Grammatical particle

n linguistics, the term particle is often employed as a useful catch-all lacking a strict definition. In general, it is understood that particles are function words that tend to be uninflected — that is, words which do not have suffixes, for example, that reflect grammatical gender, tense or person. However, the term may have a broader definition.

Depending on its context, the meaning of the term may overlap with such notions as "morpheme", "marker", or even "adverb" (another catch-all term). Like many linguistic concepts, the precise content of the notion is very language-specific.

The term particle is often used in descriptions of Japanese and Korean, where they are used to mark nouns according to their case or their role (subject, object, complement, or topic) in a sentence or clause. Some of these particles are best analysed as case markers and some as postpositions.

Under the strictest definition, which demands that a particle be an uninflected word, English deictics like this and that would not be classed as such (since they have plurals), and neither would Romance articles (since they are inflected for number and gender).

On the other hand, if a particle is defined as simply an invariable word, interjections are to be classed as particles, as well as sentence-tagging particles like Japanese and Chinese question markers.
http://en.wikipedia.org/wiki/Grammatical_particle