"spontaneous" in the adjective sense
1. spontaneous, self-generated
happening or arising without apparent external cause
"spontaneous laughter"
"spontaneous combustion"
"a spontaneous abortion"
2. ad-lib, spontaneous, unwritten
said or done without having been planned or written in advance
"he made a few ad-lib remarks"
Source: WordNet® (An amazing lexical database of English)
Princeton University "About WordNet®."
WordNet®. Princeton University. 2010.
View WordNet® License