A function word that can be used in English to form phrasal verbs WordNet 3.0 & Open English WordNet

In grammar, the term particle has a traditional meaning, as a part of speech that cannot be inflected, and a modern meaning, as a function word associated with another word or phrase, in order to impart meaning. Wikipedia

A function word Wikipedia Disambiguation

Typically short and indeclinable word with a grammatical function but no clear part of speech Wikidata

One-syllable suffixes or short words in Japanese and Korean grammar that immediately follow the modified noun, verb, adjective, or sentence. They have a wide range of grammatical functions, including the indication of a question or the speaker's assertiveness, certitude, or other feelings. OmegaWiki

A part of speech which cannot be inflected: an adverb, preposition, conjunction or interjection. Wiktionary