|Definition - The particle to when it is used to form the infinitive, e.g. to eat and to sleep.|
Example Usages -
1. "Among individual words commonly so classed are the negative particle not (and its contraction n't), the infinitival 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)."
(Source: Sidney Greenbaum, "Particle," Oxford Companion to the English Language. Oxford Univ. Press, 1992 )