“A word that does not change its form through inflection and does not fit easily into the established system of the parts of speech. Among individual words commonly so classed are the negative particle not (and its contraction n’t), the infinitive particle to (to go; to run), the imperative particles do, don’t (Do tell me; Don’t tell me) and let, let’s (Let me see now; Let’s go). There is also a set of adverbial and prepositional particles that combine with verbs to form phrasal verbs (out in look out; up in turn up) and prepositional verbs (at in get at; for in care for). The term pragmatic particle is sometimes used for words that play a role in maintaining discourse and are also known as fillers and discourse markers: oh, ah, well, yes, no, actually, anyway.”
Excerpted from: McArthur, Tom. The Oxford Concise Companion to the English Language. New York: Oxford University Press, 2005.
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