The story
From the surname of Joseph Louis François Bertrand, who conjectured the theorem, and the surname of Pafnuty Chebyshev who proved it.
Etymology from Wiktionary, CC BY-SA 4.0.
Bertrand-Chebyshev theorem
Today it means: the theorem that there is at least one prime number between n and 2n for every n>1, i.e. Full definition →