[...] "particle", in its customary broad usage, is a pretheoretical notion that has no translation into a theoretical construct of linguistics, and must be eliminated in favor of such constructs.
- Zwicky (2004), Pag. 351

[...] "particles" (i.e. function items, whether words or not) [...].
- Zwicky (2004), Pag. 363

[...] particles belong to NO syntactic category: that they are acategorial. This is equivalent to saying that these words are directly introduced by syntactic rules, rather than appearing as instances of lexical categories.
- Zwicky (2004), Pag. 361

The familiar class "Prt" of Eng. verbal "particles" - the "off" of "send off", the "up" of "give up" - is a typical set of words which get this label because no more suitable one is available. They are, first of all, semantically peculiar: their contribution to the combinations in which they occurr tends to be idiosyncratic, and in any case this contribution is not that of either of the two closest word classes in English, preposition and (directional) adverbs [...] the range of meanings for the things that have been called "particles" in one languge or another parallels exactly the range of meanings for clitics in the language of the world, and these in turn parallel exactly the range of meanings for inflectional affixes. Semantically, particles are "functions", rather than "content", items [...]. Phonologically, the things labeled as particles tend to be "dependent", again like clitics and affixes.
- Zwicky (2004), Pag. 359

The most common use of the term is to label items which, in contrast to those in established word classes of a language, have (a) peculiar semantics and (b) idiosyncratic distributions. Thus "particle" is a cover term for items that do not fit easily into syntactic and semantic generalizations about the language.
- Zwicky (2004), Pag. 358