When you have a bunch of related words - like am, is, was - then the lemma is the one you look up in the dictionary.
In the above example the lemma is, of course, be.
Identifying an English lemma is not usually as difficult as that, of course, but it can be tricky in Russian, say, where words tend to come in many different forms depending upon the work they have to do.
Lemmas can also be awkward in languages like Irish, where the first letter of some words sometimes changes.
The lemma tends to be the shortest form of the word - even if, as in Arabic, the shortest form of a verb is the third person plural masculine past tense.
The word lemma itself has two alternative plurals, lemmas and lemmata, but the word you'll find in the dictionary is lemma. And so lemma, rather neatly, is itself a lemma.
Lemma has another language-based meaning which describes the moment when someone is wanting to say something and they have the concept they need in their heads, but haven't got round to finding the actual word for it, yet.
Mind you, this sort of lemma might not actually exist.
Word To Consider Today: lemma. This word is the Greek for premise, and comes from lambanein, to take (as in to take for granted).
PS This happens to be The Word Den's two thousand, two hundred, and twenty second post.
Do feel free to celebrate.