The lemmatizer is a language-processing tool that converts words into their base form or "lemma". A lemma is the neutral form of a word, without declension or conjugation, as it appears in dictionaries. For example, in Basque, the word "etxeetan" would become the lemma "etxe," or the word "joaten" would become the lemma "joan". Lemmatization simplifies text analysis by grouping different forms of a word under a single lemma.