spontaneous

American Heritage® Dictionary of the English Language, Fourth Edition

1. adj. Happening or arising without apparent external cause; self-generated.
2. adj. Arising from a natural inclination or impulse and not from external incitement or constraint.
3. adj. Unconstrained and unstudied in manner or behavior.
4. adj. Growing without cultivation or human labor.

Century Dictionary and Cyclopedia

1. Proceeding from a conscious or unconscious internal impulse; occurring or done without the intervention of external causes; in a restricted sense, springing from one's own desire or volition, apart from any external suggestion or incitement. Of late the employment of spontaneous in the sense of ‘irreflective’ or ‘not controlled by a definite purpose’ is creeping in from the French; but this is an objectionable use of the term.
2. Growing naturally, without previous human care.
3. Growing as native; indigenous.
4. In biology, instinctive or automatic, as some actions of animals which depend upon no external stimulus and are performed without apparent motive or purpose; uninfluenced by external conditions, as a change in structural character. Compare spontaneity, 2. Spontaneous actions may be either voluntary, in a usual sense, as the gambols of puppies or kittens, or involuntary and quite uncontrollable by the will. Of the latter class, some are abnormal, as spontaneous (in distinction from induced) somnambulism, and these are also called idiopathic.
5. Synonyms Willing, etc. (see voluntary), instinctive, unbidden.

Wiktionary

1. adj. Self generated; happening without any apparent external cause.
2. adj. Done by one's own free choice, or without planning.
3. adj. proceeding from natural feeling or native tendency without external constraint
4. adj. arising from a momentary impulse
5. adj. controlled and directed internally : self-active : spontaneous movement characteristic of living things
6. adj. produced without being planted or without human labor : indigenous
7. adj. not apparently contrived or manipulated : natural
8. adj. Random.
9. adj. Sudden, without warning.

GNU Webster's 1913

1. adj. Proceeding from natural feeling, temperament, or disposition, or from a native internal proneness, readiness, or tendency, without constraint.
2. adj. Proceeding from, or acting by, internal impulse, energy, or natural law, without external force
3. adj. Produced without being planted, or without human labor.

WordNet 3.0

1. adj. said or done without having been planned or written in advance
2. adj. happening or arising without apparent external cause