Spontaneous

Late Latin spontaneus, from Latin sponte (sua) ("of one's free will, voluntarily").

Full definition of spontaneous

Self-generated; happening without any apparent external cause.He made a spontaneous offer of help.
Done by one's own free choice, or without planning.
proceeding from natural feeling or native tendency without external constraint
arising from a momentary impulse
controlled and directed internally; self-active; spontaneous movement characteristic of living things
produced without being planted or without human labor; indigenousa spontaneous growth of wood
not apparently contrived or manipulated; natural
Random.
Sudden, without warning.