means the quality of acting on impulse or natural feeling, without planning, prompting, or premeditation

from From Latin sponte, 'of one's own accord, willingly' — a word that survives mostly in fossilized phrases like sua sponte ('of its own will'). It grew into the Late Latin spontaneus, 'voluntary,' which English borrowed as 'spontaneous' in the 1600s; 'spontaneity' followed as the noun for the quality, formed with the -ity ending that turns adjectives into abstractions. At root it carries the idea of something arising from within, under no compulsion from outside.