a machine or software application that accesses a cloud over a network connection, perhaps on behalf of a consumer; and
NIST SP 800-146
A function that uses the PKI to obtain certificates and validate certificates and signatures. Client functions are present in CAs and end entities. Client functions may also be present in entities that are not certificate holders. That is, a system or user that verifies signatures and validation paths is a client, even if it does not hold a certificate itself. See section 2.4.
NIST SP 800-15 under client (or PKI client)
A system entity, usually a computer process acting on behalf of a human user, that makes use of a service provided by a server.
NIST SP 800-32 under Client (application)