stateless. A stateless server is one which treats each request as an independent transaction, unrelated to any previous request. This simplifies the server design because it does not need to allocate storage to deal with conversations in progress or worry about freeing it if a client dies in mid-transaction.
An expression in a programming language is a combination of one or more explicit values, constants, variables, operators, and functions that the programming language interprets (according to its particular rules of precedence and of association) and computes to produce (to return, in a stateful environment) another value.
An expression is basically an incomplete mathematical sentence. It is like any normal phrase in the English language. Compared to expressions, equations are more complete. They are homologous to what completely structured English sentences are.