The story
The result is due to Gregory Freiman (1964, 1966).
Etymology from Wiktionary, CC BY-SA 4.0.
Freiman's theorem
Today it means: In additive combinatorics, a central result that indicates the approximate structure of sets whose sumset is small. It roughly states that if |A+A|/|A| is small, then A can be contained in a small generalized arithmetic progression. Full definition →