say it: spahn-TAY-nee-uhs
spontaneous is an adjective meaning "happening or arising without apparent external cause". It carries 2 recorded senses.
happening or arising without apparent external cause
“spontaneous laughter”
“spontaneous combustion”
“a spontaneous abortion”
said or done without having been planned or written in advance
“he made a few ad-lib remarks”
Definitions, synonyms and semantic relations for spontaneous come from Princeton University’s WordNet. The pronunciation comes from the CMU Pronouncing Dictionary. Commonness is measured against a corpus of everyday English. Full sources and licences.