I encountered Lambda Calculus when researching how computations can be simplified into some extreme forms. Lambda Calculus expresses computation “based on function abstraction and application using variable binding and substitution” (Wikipedia). It’s Turing Complete (and personally I think it’s a more elegant model compared to the Turing Machine, from the way it abstract computation), and some people view it as the very minimal functional programming language (it definitely inspired some, like Lisp).