1. happening or arising without apparent external cause (Freq. 8) - spontaneous laughter - spontaneous combustion - a spontaneous abortion • Syn: self-generated • Ant: induced • Similar to: impulsive, unprompted, intuitive, natural, instinctive • Derivationally related forms: spontaneity 2. said or done without having been planned or written in advance - he made a few ad-lib remarks • Syn: ad-lib, unwritten • Similar to: unscripted • Derivationally related forms: spontaneousness