informal vs cosy

informal is not a noun while cosy is a noun.

Word	Noun	Adjective	Verb	Adverb
informal	No	Yes	No	No
cosy	Yes	Yes	No	No

As adjectives, informal and cosy are synonyms defined as:

informal and cosy: having or fostering a warm or friendly and informal atmosphere

Other synonyms of informal include cozy, intimate.

informal (noun)	cosy (noun)
	a padded cloth covering to keep a teapot warm

informal (adjective)	cosy (adjective)
not formal	enjoying or affording comforting warmth and shelter especially in a small space
used of spoken and written language	suggesting connivance
having or fostering a warm or friendly and informal atmosphere	having or fostering a warm or friendly and informal atmosphere
not officially recognized or controlled