spontaneous
/spän-ˈtā-nē-əs/
adjective
proceeding from natural feeling or native tendency without external constraint
arising from a momentary impulse
controlled and directed internally : self-acting
spontaneous impulsive instinctive automatic mechanical mean acting or activated without deliberation. spontaneous implies lack of prompting and connotes naturalness. impulsive implies acting under stress of emotion or spirit of the moment.
noun
self-ignition of combustible material through chemical action (such as oxidation) of its constituents —called also spontaneous ignition
noun
a now discredited notion that living organisms spontaneously originate directly from nonliving matter
A difficulty that we have forgotten lay in the widespread belief in spontaneous generation. Aristotle had written that flies, worms, and other small animals originated spontaneously from putrefying matter.