Definitions can’t extend a theory, because they don’t give you new theorems.
A conservative extension of a language/theory doesn’t introduce new theorems in the old language, but could introduce new theorems that make use of new symbols, although in the case of extension by definitions, all new theorems can also be expressed in the smaller (original) language and would be the theorems of original theory.
A conservative extension of a language/theory doesn’t introduce new theorems in the old language, but could introduce new theorems that make use of new symbols, although in the case of extension by definitions, all new theorems can also be expressed in the smaller (original) language and would be the theorems of original theory.
Okay, thanks. I didn’t know that adding certain kinds of axioms was called “extension by definitions”.