Predetermined is an adjective used to describe a plan, decision or outcome that has been established in advance. Some antonyms or opposite words for predetermined could be improvised, spontaneous, unpremeditated, unexpected, uncertain or arbitrary. These antonyms suggest the absence of premeditated actions or decisions. Improvised means a spur-of-the-moment decision or plan made on the spot. Spontaneous suggests a natural, instinctual or unplanned occurrence. Unpremeditated means an action that happened without forethought or planning. Unexpected implies a surprising or unforeseen outcome. Uncertain suggests a vague or ambiguous decision. Arbitrary means a decision with no logical or systematic basis.