adj.即兴的；adj.自主的，自发的；adj.自然的，不刻意的
Merriam-Webster Online Dictionary
spontaneous (adjective)1.
proceeding from natural feeling or tendency without external - native constraint
2.
arising from a momentary impulse
3.
controlled and directed internally - self-acting spontaneous movement characteristic of living things
4.
produced without being planted or without human labor - indigenous
5.
developing or occurring without apparent external influence, force, cause, or treatment
6.
not apparently or manipulated - contrived natural
Merriam-Webster Online Thesaurus
spontaneous (adjective)done instantly and without conscious thought or decision
SYNONYMS:
instinctive, instinctual, involuntary, knee-jerk, mechanic, mechanical, robotic, spontaneousRELATED WORDS:
conditioned, natural, Pavlovian, reactive, reflex, simple, subliminal, unconscious, unforced, visceral; blind, inadvertent, unintended, unintentional, unwilling, unwitting; abrupt, quick, ready, sudden; ad-lib, extemporaneous, extempore, impromptu, improvised, offhand, offhanded, off-the-cuff, snap, spur-of-the-moment, unconsidered, unplanned, unpremeditated, unprepared, unprompted, unreasoned, unrehearsed, unstudied; casual, chance, chancy, haphazard, hasty, hit-or-miss, impetuous, impulsive, mindless, random, rashNEAR ANTONYMS:
calculated, conscious, cultivated, deliberate, designed, intended, intentional, predetermined, prepared, projected, refined, rehearsed, volitional, voluntary, willed, willful ( wilful); advised, aforethought, careful, considered, foresighted, forethoughtful, measured, meticulous, reasoned, studied, thoughtful