spontaneous

Spontaneous is an adjective describing events or actions that arise naturally, without external prompting or premeditation. In everyday language, a spontaneous decision occurs by personal impulse rather than planning. The term comes from Latin spontaneus, meaning “of one's own accord.”

Within science, spontaneity refers to processes driven by a system's internal state. In thermodynamics, a reaction or process is described as spontaneous if it can proceed without ongoing external input, though the rate may depend on kinetics. A spontaneous change typically lowers the system's free energy (negative delta G) at the given conditions.

Historical discussions include spontaneous generation, the idea that living organisms arise from nonliving matter. This concept was debunked in the 17th–19th centuries by experiments of Redi and Pasteur, establishing that life arises from preexisting life under appropriate conditions.

Several phenomena are labeled spontaneous in common usage or scientific contexts: spontaneous combustion, where materials self-ignite due to internal heat from oxidation; spontaneous emission, the quantum process by which an excited atom emits a photon without external stimulation; spontaneous remission, the unexplained improvement or disappearance of a disease; spontaneous symmetry breaking, in which a system with symmetric laws ends up in an asymmetric state; and spontaneous order, where organized structure emerges without external directives.

Spontaneity also features in culture and arts, referring to improvisation or unplanned creative bursts.