adj. 1
Self-directed; happening or acting without external cause or instigation; proceeding from natural feeling, temperament, or momentary impulse.
He made a spontaneous offer of help.
adj. 2
Done by one's own free choice, or without planning.
adj. 3
Said or done without prior planning or prior writing.
He made spontaneous remarks in the court, after being asked an unexpected question.
adj. 4
Controlled and directed internally; self-active; spontaneous movement characteristic of living things.

Related words

Word family: spontaneity spontaneously spontaneousness
Phrases: spontaneous generation spontaneous abortion spontaneous combustion

Choose the right word

Spontaneous, Voluntary, Involuntary, Unbidden or Willing?

Thus voluntary and involuntary, which are antonyms of each other, are both partial synonyms of spontaneous. We speak of spontaneous generation, spontaneous combustion, spontaneous sympathy, an involuntary start, an unbidden tear, voluntary agreement, willing submission.

J. C. Fernald, English Synonyms and Antonyms (1914)

Example sentences

automatic Most workers get an automatic pay raise every year.
instinctive All forms of life have an instinctive urge to survive.
unforced In tennis, you'd call this an unforced error.
impulsive Impulsive people are more prone to procrastination.

Popularity over time

Word	1850 → 2019	Trend
spontaneous		steady
automatic		falling
instinctive		steady
unforced		rising
unconstrained		rising
impulsive		rising

Use in printed books, per decade from 1850 to 2019.

By length

4 letters: free¹
6 letters: abrupt², lavish²
7 letters: natural³
8 letters: unforced, unbidden
9 letters: automatic⁴, impulsive³, unwritten³, unstudied
10 letters: mechanical⁴, autonomous⁴, gratuitous⁴, unscripted³, unprompted
11 letters: instinctive³, precipitous⁴, uncompelled, subitaneous
12 letters: adventitious
13 letters: unconstrained³
14 letters: extemporaneous, unpremeditated

The small number after each word is its number of syllables (abrupt² = 2 syllables).