I very much enjoy reading the "What Is...?" column in the Notices of the AMS. Unfortunately, there seemed to be no index to this column. I have therefore created this one in the hope that it might be helpful to others as well. If I have missed an article, please let me know!
(A few years later, there now exists the following overview of the What Is Collection.)
- What is... a boundary object? — Alon Pinto, Yvonne Lai, May 2025 (HTML, DOI)
- What is... treewidth? — David Eppstein, February 2025 (HTML, DOI)
- What is... optical geometry? — Marcus C. Werner, December 2024 (HTML, DOI)
- What is... the Bateman-Horn Conjecture? — Stephan Ramon Garcia, November 2024 (HTML, DOI)
- What is... the Dowling-Wilson Conjecture? — Tom Braden, Jacob P. Matherne, and Nicholas Proudfoot, October 2024 (HTML, DOI)
- What is... a parking function? — J. Carlos Martínez Mori, September 2024 (HTML, DOI)
- What is... a Białynicki-Birula Decomposition? — Alberto Franceschini, April 2024 (HTML, DOI)
- What is... a skew brace? — Leandro Vendramin, January 2024 (HTML, DOI)
- What is... an inductive mean? — Frank Nielsen, December 2023 (HTML, DOI)
- What is... a metaplectic group? — Martin H. Weissman, May 2023 (HTML, DOI)
- What is... an equivariant neural network? — Lek-Heng Lim and Bradley J. Nelson, April 2023 (HTML, DOI)
- What is... a graphical design? — Catherine Babecki, October 2022 (HTML, DOI)
- What is... the Dirac equation? — M. Burak Erdoğan, William R. Green, and Ebru Toprak, November 2021
- What is... a companion? — Anna Cadoret, August 2021
- What is... the VC-Dimension? — Samuel C. Tenka, April 2021
- What is... a hyperpolygon? — Steven Rayan and Laura P. Schaposnik, January 2021
- What is... a minuscule representation? — Richard M. Green, June/July 2020
- What is... a hereditarily indecomposable Banach space? — Hossein Hosseini Giv, October 2019
- What is... a Markov basis? — Sonja Petrović, August 2019
- What is... a thin group? — Alex Kontorovich, D. Darren Long, Alexander Lubotzky, and Alan W. Reid, June/July 2019
- What is... a Coleman integral? — Jennifer S. Balakrishnan, May 2019
- What is... Falconer's conjecture? — Alex Iosevich, April 2019
- What is... a multilayer network? — Mason A. Porter, December 2018
- What is... a Borel reduction? — Matthew Foreman, November 2018
- What is... a rectifiable set? — Editors, October 2018
- What is... a matroidal family of graphs? — J. M. S. Simões-Pereira, September 2018
- What is... Riemannian holonomy? — Jacob Gross, August 2018
- What is...the Langlands Program? — Solomon Friedberg, June/July 2018
- What is...a hyperbolic 3-manifold? — Colin Adams, May 2018
- What is... hypoellipticity? — Editors, April 2018
- What is...an information projection? — Frank Nielsen, March 2018
- What is...the amplituhedron? — Jacob Bourjaily and Hugh Thomas, February 2018
- What is...an acylindrical group action? — Thomas Koberda, January 2018
- What is...a scheme? — Tom Gannon, December 2017
- What is...a varifold? — Editors, November 2017
- What is... perimeter? — Editors, October 2017
- What is...a Sobolev orthogonal polynomial? — Francisco Marcellán and Juan J. Moreno-Balcázar, September 2017
- What is...a CR submanifold? — Phillip S. Harrington and Andrew Raich, August 2017
- What is...a generalised mean-curvature flow? — Hui Yu, June/July 2017
- What is...a virtual knot? — Daniel S. Silver, Susan G. Williams, May 2017
- What is... tropical geometry? — Eric Katz, April 2017
- What is... an elliptic curve? — Harris B. Daniels and Álvaro Lozano-Robledo, March 2017
- What is... Benford's Law? — Arno Berger, Theodore P. Hill, February 2017
- What is...a complex symmetric operator? — Stephan Ramon Garcia, January 2017
- What is... symplectic geometry? — Tara S. Holm, December 2016
- What is...a blender? — Ch. Bonatti, S. Crovisier, L. J. Díaz, A. Wilkinson, November 2016
- What is...a multiple orthogonal polynomial? — Andrei Martínez-Finkelshtein and Walter Van Assche, October 2016
- What is...a Leavitt path algebra? — Gene Abrams, September 2016
- What is...a Diophantine m-tuple? — Andrej Dujella, August 2016
- What is... aperiodic order? — Michael Baake, David Damanik and Uwe Grimm, June/July 2016
- What is... Nash equilibrium? — Rajiv Sethi and Jörgen Weibull, May 2016
- What is...a quandle? — Sam Nelson, April 2016
- What is...an anabelian scheme? — Kirsten Wickelgren, March 2016
- What is... Gauss curvature? — Editors, February 2016
- What is...an ergodic transformation? — Cesar E. Silva, January 2016
- What is...an automatic sequence? — Eric Rowland, March 2015
- What is...a graphon? — Daniel Glasscock, January 2015
- What is...a perfectoid space? — Bhargav Bhatt, October 2014
- What is...a period? — Stefan Müller-Stach, September 2014
- What is...a Rauzy fractal? — Pierre Arnoux, Edmund Harriss, August 2014
- What is...a spectrahedron? — Cynthia Vinzant, May 2014
- What is... cyclic sieving? — Victor Reiner, Dennis Stanton, Dennis White, February 2014
- What is...a flag algebra? — Alexander A. Razborov, November 2013
- What is...the Leech lattice? — Chuanming Zong, October 2013
- What is...the p-adic Mandelbrot set? — Joseph H. Silverman, September 2013
- What is...a frame? — Christopher Heil, June/July 2013
- What is...a tensegrity? — Robert Connelly, January 2013
- What is...a Shimura variety? — James S. Milne, December 2012
- What is... essential dimension? — Zinovy Reichstein, November 2012
- What is...a scissors congruence? — Johan L. Dupont, October 2012
- What is... cop number? — Anthony Bonato, September 2012
- What is...an octopus decomposition? — Larry Conlon, August 2012
- What is...a biholomorphic mapping? — Eric Bedford, June/July 2012
- What is...an approximate group? — Ben Green, May 2012
- What is... data mining? — Mauro Maggioni, April 2012
- What is...an affine sphere? — Daniel J. F. Fox, March 2012
- What is...a pseudoconvex domain? — R. Michael Range, February 2012
- What is...a Fenchel conjugate? — Heinz H. Bauschke, Yves Lucet, January 2012
- What is...a natural proof? — Timothy Y. Chow, December 2011
- What is...a stationary measure? — Alex Furman, October 2011
- What is... Thompson's group? — J. W. Cannon, W. J. Floyd, September 2011
- What is...a derived stack? — Gabriele Vezzosi, August 2011
- What is...a resplendent structure? — Roman Kossak, June/July 2011
- What is...a infra-nilmanifold endomorphism?? — Karel Dekimpe, May 2011
- What is...a G2-manifold? — Spiro Karigiannis, April 2011
- What is... equivariant cohomology? — Loring W. Tu, March 2011
- What is...a free cumulant? — Jonathan Novak, Piotr Sniady, February 2011
- What is... persistent homology? — Shmuel Weinberger, January 2011
- What is...a mock modular form? — Amanda Folsom, December 2010
- What is... cost? — Damien Gaboriau, November 2010
- What is...a linear algebraic group? — Skip Garibaldi, October 2010
- What is...a sandpile? — Lionel Levine, James Propp, September 2010
- What is...a paraproduct? — Árpád Bényi, Diego Maldonado, Virginia Naibo, August 2010
- What is...a kinetic solution for degenerate parabolic-hyperbolic equations? — Gui-Qiang G. Chen, Benoît Perthame, June/July 2010
- What is...a perverse sheaf? — Mark Andrea de Cataldo, Luca Migliorini, May 2010
- What is...a Wilf-Zeilberger pair? — Akalu Tefera, April 2010
- What is...a Gaussian entire function? — Fedor Nazarov, Mikhail Sodin, March 2010
- What is...a halting probability? — Cristian S. Calude, G. J. Chaitin, February 2010
- What is...an Oka manifold? — Finnur Larusson, January 2010
- What is...a Rota-Baxter algebra? — Li Guo, December 2009
- What is...a Legendrian knot? — Joshua M. Sabloff, November 2009
- What is...Stanley depth? — M. R. Pournaki, S. A. Seyed Fakhari, M. Tousi, S. Yassemi, October 2009
- What is...the complex dual to the real sphere? — Simon Gindikin, September 2009
- What is...the parity phenomenon? — John Friedlander, Henryk Iwaniec, August 2009
- What is...an elliptic genus? — Serge Ochanine, June/July 2009
- What is...measure rigidity? — Manfred Einsiedler, May 2009
- What is...Lehmer's number? — Eriko Hironaka, March 2009
- What is...a skein module? — W. B. Raymond Lickorish, February 2009
- What is...the Schwarzian derivative? — Valentin Ovsienko, Sergei Tabachnikov, January 2009
- What is...a period domain? — James Carlson, Phillip Griffiths, December 2008
- What is...a cross ratio? — François Labourie, November 2008
- What is...stable commutator length? — Danny Calegari, October 2008
- What is...an ∞-category? — Jacob Lurie, September 2008
- What is...outer space? — Karen Vogtmann, August 2008
- What is...forcing? — Thomas Jech, June/July 2008
- What is...a toric variety? — Ezra Miller, May 2008
- What is...property A? — Piotr Nowak, Guoliang Yu, April 2008
- What is...a systole? — Marcel Berger, March 2008
- What is...JPEG? — David Austin, February 2008
- What is...quantum chaos? — Ze'ev Rudnick, January 2008
- What is...a cluster algebra? — Andrei Zelevinsky, December 2007
- What is...Boy's surface? — Robion Kirby, November 2007
- What is...a Woodin cardinal? — John Steel, October 2007
- What is...a Higgs bundle? — Steve Bradlow, Oscar García-Prada, Peter Gothen, September 2007
- What is...a Galois representation? — Mark Kisin, June/July 2007
- What is...an infinite swindle? — Valentin Poénaru, May 2007
- What is...a tropical curve? — Grigory Mikhalkin, April 2007
- What is...a minimal model? — János Kollár, March 2007
- What is...a Young tableau? — Alexander Yong, February 2007
- What is...a projective structure? — William Goldman, January 2007
- What is...a quasiconformal mapping? — Juha Heinonen, December 2006
- What is...Turing reducibility? — Martin Davis, November 2006
- What is...a bad end? — Charles Epstein, October 2006
- What is...a quasicrystal? — Marjorie Senechal, September 2006
- What is...a strange attractor? — David Ruelle, August 2006
- What is...a coarse space? — John Roe, June/July 2006
- What is...percolation? — Harry Kesten, May 2006
- What is...a syzygy? — Roger Wiegand, April 2006
- What is...a graph minor? — Bojan Mohar, March 2006
- What is...a quantum group? — Shahn Majid, January 2006
- What is...a random matrix? — Persi Diaconis, December 2005
- What is...a Gröbner basis? — Bernd Sturmfels, November 2005
- What is...a pseudoholomorphic curve? — Simon Donaldson, October 2005
- What is...a Lefschetz pencil? — Robert Gompf, September 2005
- What is...a compacton? — Philip Rosenau, August 2005
- What is...property tau? — Alex Lubotzky, June/July 2005
- What is...a horseshoe? — Mike Shub, May 2005
- What is...a dimer? — Richard Kenyon, Andrei Okounkov, March 2005
- What is...a brane? — Gregory Moore, February 2005
- What is...an open book? — Emmanuel Giroux, January 2005
- What is...a flip? — Alessio Corti, December 2004
- What is...a motive? — Barry Mazur, November 2004
- What is...a topos? — Luc Illusie, October 2004
- What is...a grope? — Peter Teichner, September 2004
- What is...an expander? — Peter Sarnak, August 2004
- What is...an operad? — James Stasheff, June/July 2004
- What is...a free lunch? — Fredy Delbaen, Walter Schachermayer, May 2004
- What is...a billiard? — Yakov Sinai, April 2004
- What is...a quasi-morphism? — D. Kotschick, February 2004
- What is...a curvelet? — Emmanuel Candès, December 2003
- What is...a dessin d'enfant? — Leonardo Zapponi, August 2003
- What is...a bubble tree? — Thomas Parker, June/July 2003
- What is...a worm? — Harold Boas, May 2003
- What is...a stack? — Dan Edidin, April 2003
- What is...a train track? — Lee Mosher, March 2003
- What is...a gerbe? — Nigel Hitchin, February 2003
- What is...a shtuka? — David Goss, January 2003
- What is...an alteration? — Frans Oort, December 2002
- What is...a building? — Kenneth S. Brown, November 2002
- What is...the monster? — Richard Borcherds, October 2002
- What is...an amoeba? — Oleg Viro, September 2002