Learn in the textbookPKI and the Core of Public Key Infrastructure
DefinitionA client certificate (クライアント証明書) is a certificate that shows the user or device connecting to a server is a legitimate party.
ExplanationIt is installed in advance on the devices of authorized users, and the server checks it on every connection. Used together with an ID and password, it keeps out devices without the certificate even if someone knows the password.
AnalogyIt is like a membership card at a members-only club: the staff check each arriving guest's card to confirm they are a real member.
Memory hookA client is the one who asks for a service, so this is the certificate held by the requesting side.