A 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 declensions or verb conjugations, as it appears in dictionaries. For example, in Basque, the word 'etxeetan' would be converted into the lemma 'etxe', or the word 'joaten' into 'joan'. Lemmatization simplifies text analysis by grouping different word variants under a single lemma.