According to wikipedia:
"In mathematics, an expression or mathematical expression is a finite combination of symbols that is well-formed according to rules that depend on the context. Mathematical symbols can designate numbers (constants), variables, operations, functions, punctuation, grouping, and other aspects of logical syntax."
https://en.wikipedia.org/wiki/Expression_(mathematics)
To me, this definition seems kind of vague, I wonder if there exists a precise definition of "mathematical expression".