A lemma is the canonical form of a word, also known as the dictionary form because it is often used as the primary entry in a dictionary. For example, the word write would be the lemma for the words writing, written, and wrote. Lemmas are useful in natural language processing (NLP) when the goal is to reduce all words in a text to their base form so that they can be analyzed as a set. The process of reducing words to their lemmas is known as lemmatization.

Unlike word stems, lemmas only remove inflectional endings. As a result, lemmas tend to more accurately reflect the meaning of the original word, whereas meaning may be lost in a pure stem.

In mathematics, a lemma is a minor proposition that is already proven and can be used in an auxiliary fashion to prove some greater proposition.