adjective

1. preceded by two others in a series; 3d or 3rd
2. next below the second in rank, power, value, merit, excellence, etc.
3. designating any of the three equal parts of something

Etymology: ME thirde, altered by metathesis < thridde < OE thridda < IE *tṛtiyo- (< base *trei-, three) > L tertius, Gr tritos

adverb

noun

1. the one following the second
2. any person, thing, class, place, etc. that is third
3. any of the three equal parts of something;
4. the third forward gear of a transmission: it provides more speed but less torque than second
5. Baseball third base
6. Music
   1. the third tone of an ascending diatonic scale, or a tone two degrees above or below any given tone in such a scale; mediant
   2. the interval between two such tones, or a combination of them