[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.