hot line vs hotline

hot line and hotline both are nouns.

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

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

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

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