Of or relating to a third party.

(video games) To intervene in a fight between two players or teams, often allowing one to easy defeat both (sometimes seen as unfair).

(US, education) The period in school that comes after second grade and before fourth grade.

(music) a major third in the final tonic chord of a minor-key passage, a device mostly used in Baroque, although occasionally in Classical music, and then adopted much later on in neoclassicism.

(idiomatic) A person or thing that serves no useful purpose. / (idiomatic) An unwanted third party accompanying two people on a date.

An approach or solution which strives for an alternative to or middle ground between two opposing viewpoints.

(American football) The third down in a series that has a maximum of four downs; traditionally the last running/passing play before punting.

Alternative letter-case form of Third World

(law) The least severe level of felony, typically involving a lack of planning and/or the presence of various mitigating factors.

(originally) The caste of commoners in France prior to 1789. / (by extension) The commoners, collectively, in any society. / (politics) The House of Commons.