1. Following the second position in an ordering or series:
2. The musical interval between one note and another three notes away from it
In music, an interval of three diatonic notes.
A third is an “imperfect interval” (having “major” and “minor” variants). A major third consists of four semitones (for example, c–e). A minor third consists of three semitones (c–e flat).

ETYM Old Eng. thirde, as. thridda, from thrî, threó, three; akin to Dutch derde third, German dritte, Icel. thrithi, Goth. thridja, Latin tertius, Greek tritos, Skr. tortîya. Related to Three, Riding a jurisdiction, Tierce.
Being one of three equal parts.
Coming next after the second and just before the fourth in position; SYN. 3rd, tertiary.

In the third place; SYN. thirdly.