The expressive power, or
expressiveness, of a
language is the extent to which it is able to express a wide range of meanings. What kind of meaning obviously depends on the language.
This term is useful for languages of which both syntax and semantics are formally defined, that is, mathematically.
In that case, the meaning of the language has a precise definition, and the comparison of different languages with respect to expressive power becomes a precise mathematical issue.
For example, regular expressions are languages that define sets of strings (the strings that 'match' the expression). Many different variants of regular expressions exist, and some variants can express sets of strings that other variants cannot.