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.