The claim that every elliptic curve is also a modular form in disguise.

Also called the Shimura-Taniyama-Wiles Conjecture, STW is an ambitious attempt to connect two apparently unrelated areas of mathematics: the theory of numbers and the theory of shapes.

I find amusing one commentator's attempt to clarify this mindbendingly esoteric topic:

"Only front-line mathematicians will really understand the STW conjecture. But you could say 'there exists a modular form of weight two and level N which is an Eigenform under the Hecke series and has a Fourier series.'"

STW was used by Andrew Wiles as part of his 1993 proof of Fermat's Last Theorem.