Which is correct: sign in, signin or sign-in when used as a noun and also as a verb?
The verb is sign in.
The noun is sign-in.
The noun is better with the hyphen, because signin could be confused with an abbreviated signing that's lost its apostrophe.
Alternatively you could use the more common log in for the verb, and login for the noun.
The correct version of the verb is sign in. Alternatively, you can use log in.