Before we dive into evaluation semantics, we need to understand the concept of a value in the lambda-calculus. What do you get when we successfully evaluate a lambda term? Well, another lambda term, but what terms are considered as values? To answer this question, we present the following definitions.
It is pretty evident why values ought to be abstractions. But why closed? One can think of closed abstractions as constants, since free variables in a lambda term can be analogous to variables in elementary algebraic expressions. Values in lambda calculus are also known as combinators, as seen in past lectures.