"The subject of K -theory takes its name from a 1957 construction of
Alexander Grothendieck which appeared in the
Grothendieck–Riemann–Roch theorem, his generalization of
Hirzebruch's theorem.[2] Let X be a smooth algebraic variety.
To each vector bundle on X , Grothendieck associates an invariant, its class .
The set of all classes on X was called K(X) from the German Klasse ."
— Wikipedia, Algebraic K -theory
In memory of a mathematician who reportedly died on July 3rd, 2021 —
For a somewhat simpler K , see Aesthetic Distance.