I am looking for an accurate and understandable definition. The ones I have found differ from each other:
Denotational semantics is a mathematical expression of the formal meaning of a programming language.
Denotational semantics is an approach of formalizing the meanings of programming languages by constructing mathematical objects (called denotations) that describe the meanings of expressions from the languages
The term "denotational semantics" refers to both the mathematical meanings of programs and the approach of giving such meanings to programs. It is like, say, the word "history", which means the history of something as well as the entire research field on histories of things.