A part-of-speech (PoS) tagger is a software tool that labels words as one of several categories to identify the word's function in a given language. In the English language, words fall into one of eight or nine parts of speech. Part-of-speech categories include noun, verb, article, adjective, preposition, pronoun, adverb, conjunction and interjection.
PoS taggers use algorithms to label terms in text bodies. These taggers make more complex categories than those defined as basic PoS, with tags such as “noun-plural” or even more complex labels. Part-of-speech categorization is taught to school-age children in English grammar, where children perform basic PoS tagging as part of their education.
PoS taggers categorize terms in PoS types by their relational position in a phrase, relationship with nearby terms and by the word’s definition. PoS taggers fall into those that use stochastic methods, those based on probability and those which are rule-based.
One of the first PoS taggers developed was the E. Brill tagger, a rule-based tagging tool. E. Brill is still commonly used today. Other tools that perform PoS tagging include Stanford Log-linear Part-Of-Speech Tagger, Tree Tagger, and Microsoft’s POS Tagger. Part-of-speech tagging is also referred to as word category disambiguation or grammatical tagging.