the.com/signin
The velvet rope between you and your own stuff, guarded by a password you forgot.
means The act of proving you are you to a system, usually with a username and password, to access an account.
from From the literal act of signing a register on entry; the closed-up noun form bloomed online to label the screen where credentials are checked.
Noun trapSign in is the verb; signin is the noun or button.
Quiet cousinOften used interchangeably with the older login.
Single sign-onOne login unlocking many services, abbreviated SSO.