Lemma Means "auxiliary theorem." The word is of
Greek origin; a more literal translation would be "what
is assumed."
     We are trying to prove a theorem, say, A. We are led 
to suspect another theorem, say B. If B were true we could
perhaps, using it, prove A. We assume B provisionally
postponing its proof, and go ahead with the proof of A.
Such a theorem B is assumed, and is auxiliary theorem 
to the original proposed theorem A. Our little story 
is fairly typical and explained the present meaning of the
word "lemma."