adj. 1 acting or done or occurring without external cause. 2 voluntary, without external incitement (made a spontaneous offer of his services). 3 Biol. (of structural changes in plants and muscular activity esp. in young animals) instinctive, automatic, prompted by no motive. 4 (of bodily movement, literary style, etc.) gracefully natural and unconstrained. 5 (of sudden movement etc.) involuntary, not due to conscious volition. 6 growing naturally without cultivation. spontaneous combustion the ignition of a mineral or vegetable substance (e.g. a heap of rags soaked with oil, a mass of wet coal) from heat engendered within itself, usu. by rapid oxidation. spontaneous generation the supposed production of living from non-living matter as inferred from the appearance of life (due in fact to bacteria etc.) in some infusions; abiogenesis. spontaneous suggestion suggestion from association of ideas without conscious volition. spontaneity n. spontaneously adv. spontaneousness n.
[ LL spontaneus f. sponte of one's own accord ]