adjective

1. acting in accordance with or resulting from a natural feeling, impulse, or tendency, without any constraint, effort, or premeditation
2. having no apparent external cause or influence; occurring or produced by its own energy, force, etc. or through internal causes; self-acting
3. growing naturally without being planted or tended; indigenous; wild

Etymology: LL spontaneus < L sponte, of free will < IE base *(s)pen(d)-, to pull > spin