I've never heard of either word, although apparently lemma is a mathematical term. Where did you see these words? Could you tell us the sentences they appeared in?

lemma: a subsidiary proposition that is assumed to be true in order to prove another proposition

I cann't find these two words in my dictionary, so Can you tell us where they encounter you.

a laxeme is a team and the lemma is for the captain.

oh I see, your question is about lingustics. I can answer it in Chinese but my English is so poor to express that. but I will have a try. A lexeme is an abstract unit , that corresponds to a set of forms taken by a single word. For example, run, runs, ran and running are forms of the same lexeme. and a lemma is the citation form in the dictionary, of this set of lexeme. I should call "basic form' a lexeme refers to a set of words, and the lemma is the first one of them.