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.
(A few years later, there now exists the following overview of the What Is Collection.)
- 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