verb: 'To opt' is to make a choice or a decision. To 'opt for' something is to choose it.
Usually opt is used in the phrases opt-in, opt-out, opt for, and co-opt.
It may be used outside one of these phrases ("I opt not to try"), but it sounds a little weird to today's English speakers. It absolutely cannot be used as a command ("Opt already!"), and generally cannot be used to replace 'choose' ("You'd better opt quickly" / "make an opt"). Opt is likely to be one of the next words to drop out of the English language as a unbound morpheme. I for one will miss it.
It entered our language a mere 200 years ago (in 1877), coming from the French opter, meaning "to choose".
Opt. Is also an abbreviation for optative, optical, options, optional, and overprint (in stamp collecting).