The adjective indefinite describes something that is vague or not clearly defined: “After injuring her knee in a riding accident, Gloria postponed her vacation for an indefinite period of time.”
You’ve probably noticed that a big chunk of the word indefinite is made up of a familiar word, definite — which means “specific” or “clearly defined.” The two words have common origins in the Latin word definitus (“definite”), but indefinite’s in- prefix gives the word the opposite meaning — "not clearly defined". Indefinite, wonderfully vague and undefined, is your friend for those times when your plans are up in the air (“indefinite plans”) or you don’t want to be pinned down to any particular timeframe (“on indefinite hiatus”).