Learn in the textbookAdvanced Authentication Methods and Payment Authentication
DefinitionSingle sign-on (シングルサインオン, SSO) lets a user authenticate once at the start and then reach multiple services they are permitted to use without signing in again.
ExplanationUsers no longer type an ID and password for each service, and they have fewer passwords to remember. The flip side is that if the first authentication is broken, an intruder gets into every linked service, so the entry point should be hardened with multi-factor authentication or similar.
AnalogyIt is like an amusement park where your day pass is checked once at the gate, and you then ride every attraction inside without buying another ticket.
Memory hookSign-on is just another word for logging in, so the name reads literally: a single sign-on (login) is all it takes.