hotline vs hot line

hotline and hot line both are nouns.

Word	Noun	Adjective	Verb	Adverb
hotline	Yes	No	No	No
hot line	Yes	No	No	No

As nouns, hotline and hot line are synonyms defined as:

hotline and hot line: a telefone line that provides direct and prompt information or advice to the caller in relation to a specific subject

hotline (noun)	hot line (noun)
a telefone line that provides direct and prompt information or advice to the caller in relation to a specific subject	a direct telephone line between two officials
	a telefone line that provides direct and prompt information or advice to the caller in relation to a specific subject