As a Verb or Adjective: 1. To be the thing, person, event or period of time that happens or comes after two others of the same kind. English = (be) third. 2. To be the third in order, the third in importance, or in the third stage of development (especially in education). Formal English = (be) tertiary. As Modifier: 1. Used immediately next to a verb to mean the action is done after two others have done it. English = third. 2. Used at the beginning of a sentence after you have started to mention several things (events, reasons, etc.) and have got to your number three point. English = third, thirdly.