An application making protected resource requests with the resource owner’s authorization and on the requesting party’s behalf.
Note that the “requesting party” is a defined term:
An end-user, or a corporation or other legal person, that uses a client to seek access to a protected resource. The requesting party may or may not be the same party as the resource owner.
and also that “resource owner” is a defined term, and does not have to “own” the resource:
resource owner An entity capable of granting access to a protected resource. When the resource owner is a person, it is referred to as an end- user.
UMA (draft-06) definition of the “client” is much better than RFC6749. It is something that needs to be considered for an early revision or errata, IMHO.