\begin{definition}[Definition:Lemma]
A '''lemma''' is a statement which is proven during the course of reaching the proof of a theorem.
Logically there is no qualitative difference between a '''lemma''' and a theorem.
They are both statements whose value is either true or false.
However, a '''lemma''' is seen more as a stepping-stone than a theorem in itself (and frequently takes a lot more work to prove than the theorem to which it leads).
Some lemmas are famous enough to be named after the mathematician who proved them (for example: Abel's Lemma and Urysohn's Lemma), but they are still categorised as second-class citizens in the aristocracy of mathematics.
:''Always the lemma, never the theorem.''
\end{definition}