The story
Introduced in 1998 by Simon Donaldson and Richard Thomas.
Etymology from Wiktionary, CC BY-SA 4.0.
Donaldson-Thomas invariant
Today it means: Given a compact moduli space of sheaves on a Calabi-Yau threefold, its Donaldson-Thomas invariant is the virtual number of its points, i.e., the integral of the cohomology class 1 against the virtual fundamental class. Full definition →