any of the classes into which words in some languages, as Latin and English, have traditionally been divided on the basis of their meaning, form, or syntactic function, as, in English, noun, pronoun, verb, adverb, adjective, preposition, conjunction, and interjection.
part of speech is dependant on function within the sentence, not the other way around.
My favorite part of speech is the group of little guys known as modal auxiliaries or informally, modals.
Because verbs comprise the principal part of speech that denotes action, they require particular attention.
British Dictionary definitions for part of speech
part of speech
a class of words sharing important syntactic or semantic features; a group of words in a language that may occur in similar positions or fulfil similar functions in a sentence. The chief parts of speech in English are noun, pronoun, adjective, determiner, adverb, verb, preposition, conjunction, and interjection POS