a spontaneous display of affectionSyn:unplanned, unpremeditated, unrehearsed, impulsive, impetuous, unstudied, impromptu, spur-of-the-moment, extempore, extemporaneous; unforced, voluntary, unconstrained, unprompted, unbidden, unsolicited; informal off-the-cuffAnt:planned, calculated2)
a spontaneous reaction to dangerSyn:reflex, automatic, mechanical, natural, knee-jerk, involuntary, unthinking, unconscious, instinctive, instinctual, visceral; informal gutAnt:conscious, voluntary3)
a spontaneous kind of personSyn:natural, uninhibited, relaxed, unselfconscious, unaffected, open, genuine, easy, free and easy; impulsive, impetuousAnt:inhibited••spontaneous, impromptu, improvised, impulsive, offhand, spontaneous, unpremeditatedIf you're the kind of person who acts first and thinks about it later, your friends are likely to describe you as spontaneous, which means that you behave in a very natural way, without prompting or premeditation (a spontaneous embrace; a spontaneous burst of applause). Or they may call you impulsive, which has somewhat less positive connotations, suggesting someone who is governed by his or her own moods and whims without regard for others. Although impulsive behavior may be admirable (his impulsive generosity prompted him to empty his pockets), it is just as likely to be ugly or disruptive (impulsive buying; an impulsive temper). Offhand also has negative overtones, implying behavior that is spontaneous to the point of being cavalier or brusque (her offhand remarks offended them). Unpremeditated is a more formal term, often used in a legal context to describe an impulsive crime committed without forethought (unpremeditated murder). In the world of public speaking, an extemporaneous speech is one that is delivered without referring to a written text, although the speaker may have been aware that he or she would be called upon to speak, while an impromptu speech is one that the speaker was not expecting to give. Improvised is often used in the context of a musical or theatrical performance, suggesting a basic structure within which the performers are free to play in a spontaneous manner (by its very nature, jazz is improvised). But it has broader applications as well; in fact, anything that is devised on the spur of the moment may be described as improvised.
Thesaurus of popular words. 2014.