A logical ‘definition’, formulated as a biconditional statement, allows you to infer in all cases what is and isn’t in the class defined.
It does not, however, necessarily coincide with the ‘meaning’ of a concept. To use Quine’s example: the groups of “creatures with kidneys” and “creatures with hearts” may 100% overlap, such that if you know a creature has a heart, you know it has a kidney, and vice versa. But that clearly does not mean the two phrases are synonymous.
This is basically just an elucidation of things I’ve written in here in the past.