I define an enterprise's architecture as the structure of its data (both computerized and not), the structure of its processes (both computerized and not), and the interactions between the two, informed by the enterprise's goals and objectives.
I define the EA's job as creating (for start-ups only), documenting, rationalizing, optimizing, educating about, evangelizing about, and consulting about that architecture. The EA should also have authority and responsibility for architectural oversight of all creation of, and changes to, the enterprise's data and processes -- the "bones" of its architecture -- to ensure that those data and processes stay consistent with the enterprise's architecture, or that the architecture is adapted to accommodate them. But the EA should not be responsible for those changes (except to the enterprise's architecture); that's for solution architects (both computerized and not). The EA should, however, be a valued consultant in the process, due to his/her holistic view of the enterprise from an architectural point of view. Note that none of the above is specific to IT.
I have identified three major EA killers -- conflating EA with IT, with "transformation", and with the entire enterprise including its strategy.
President at XTRAN, LLC