The definition of sexp from the EmacsManual:

The word ‘sexp’ is derived from “s-expression”, the ancient term for an expression in Lisp. But in Emacs, the notion of ‘sexp’ is not limited to Lisp. It refers to an expression in whatever language your program is written in. Each programming language has its own major mode, which customizes the syntax tables so that expressions in that language count as sexps.

Here are a few EmacsLisp sexp examples:

 (foo bar 42)
 '(foo bar)