Lambda

Alonzo Church’s lambda calculus, developed in the 1930s, made λ the symbol of functions in logic and computer science. Many programming languages still call a function without a name a lambda.

Church explained the letter differently at different times. In a 1964 letter he said he started from the hat notation of Whitehead and Russell, changed the hat to ∧, then to λ for ease of printing. Later he said the choice was more or less accidental, and once replied to the question with a postcard reading “eeny, meeny, miny, moe”.