Lemma (linguistics)
In computational linguistics, a stem is the part of the word that never changes even when different forms of the word are used. A lemma is the base form of the verb. For example, from "produced", the lemma is "produce", but the stem is "produc-". This is because there are words such as production. When sound (phonology) is taken into account, the definition of the unchangeable part of the word is not so useful. Notice the sound of the words in the example: "produced"  versus "production" .