ad-lib vs spontaneous

ad-lib is a noun but spontaneous is not a noun.

Word	Noun	Adjective	Verb	Adverb
ad-lib	Yes	Yes	Yes	No
spontaneous	No	Yes	No	No

As adjectives, ad-lib and spontaneous are synonyms defined as:

ad-lib and spontaneous: said or done without having been planned or written in advance

Other synonyms of ad-lib include unwritten.

ad-lib (noun)	spontaneous (noun)
remark made spontaneously without prior preparation

ad-lib (adjective)	spontaneous (adjective)
with little or no preparation or forethought	happening or arising without apparent external cause
said or done without having been planned or written in advance	said or done without having been planned or written in advance

ad-lib (verb)	spontaneous (verb)
perform without preparation