Theorem、Proposition、Lemma的区别

第一次看英文论文，慢的很……遇到了这几个乍一看好像差不多的名词（其实差别还是蛮大的）：

Theorem：( technical 术语) a rule or principle, especially in mathematics, that can be proved to be true （尤指数学）定理

Proposition：a statement of a theorem , and an explanation of how it can be proved 命题

Lemma：( technical 术语) a statement that is assumed to be true in order to test the truth of another statement引理；辅助定理