Definition: Theorem, Lemma, Proposition, Corollary, Postulate, Statement, Fact, Observation, Expression, Fact, Property, Conjecture and Principle
Most of the time a mathematical statement is classified with one the words listed above.
However, I can't seem to find definitions of them all online, so I will request your aid in describe/define them.
Also, when is a mathematical statement a theorem versus a lemma ? I've read that a theorem is important while a lemma is not so important and used to prove a theorem. However a theorem is sometimes used to prove some other theorem. This implies that some theorems are also lemmas ?
Is it subjective with respect to the author, which statements become a theorem, lemma, etc. ?