A POS tagger is a tool used in natural language processing that labels words in a sentence with their corresponding Part of Speech (POS) tags. There are several synonyms for POS tagger that are commonly used in the field of NLP. These include part-of-speech tagger, morphosyntactic tagger, grammatical tagger, and linguistic tagger. Each of these terms refers to the same basic function of identifying the grammatical category of each word in a sentence. POS tagging is essential for several NLP tasks such as sentiment analysis, named entity recognition, machine translation, and text summarization, among others.