Particle

Particle is a somewhat nebulous term for a variety of small words that do not conveniently fit into other classes of words. The Concise Oxford Companion to the English Language defines a particle as a "word that does not change its form through inflection and does not fit easily into the established system of parts of speech". The term includes the "adverbial particles" like up or out in verbal idioms (phrasal verbs) such as look up or knock out; it is also used to include the "infinitival particle" to, the "negative particle" not, the "imperative particles" do and let, and sometimes "pragmatic particles" like oh and well. Another example is the word so in the adverb so far.