[1] RFC 5246 - The Transport Layer Security (TLS) Protocol Version 1.2 ( 版) <http://tools.ietf.org/html/rfc5246#page-79>
client
The application entity that initiates a TLS connection to a
server. This may or may not imply that the client initiated the
underlying transport connection. The primary operational
difference between the server and client is that the server is
generally authenticated, while the client is only optionally
authenticated.
[2] RFC 6125 - Representation and Verification of Domain-Based Application Service Identity within Internet Public Key Infrastructure Using X.509 (PKIX) Certificates in the Context of Transport Layer Security (TLS) ( 版) <https://tools.ietf.org/html/rfc6125#section-1.8>
TLS client: An entity that assumes the role of a client in a
Transport Layer Security [TLS] negotiation. In this specification
we generally assume that the TLS client is an (interactive or
automated) application client; however, in application protocols
that enable server-to-server communication, the TLS client could
be a peer application service.