Enterprise Architecture as Defined by Roger Sessions
Roger Sessions gave a definition of enterprise architecture on today's .net rocks podcast that I thought was quite good and along the line of what I feel is the definition:
"The discipline of bringing together the business and technology groups to provide business value."

I'm often asked what enterprise architecture is, and I'm always surprised that most think it is the way that information systems fit together, always evolving down to some technology issues.

My perspective that this has always been a problem of products and using technology to provide tools for the business to perform more effectively by having the tools support business needs.  It brings into play the technology underpinnings, but only as a means to addressing business requirements and business agility (not software agility, but I'll comment on that shortly), but it also includes management of products and building product platforms through reusable product architecture supported by proper technology architecture.

On the point of agility, one must understand that software agility (such as XP, SCRUM, …) are methodologies for providing implementation of business requirements in a satisfactory turn around time.  I'm not trying to belittle these methodologies, but it is important in my mind that the technologist understand that these methodologies are implementation details at the business level.

The point is that a company can use agile software techniques, but without proper product platform architecture, and agility in actual product management, the organization as a whole will not be agile and therefore agile software techniques, perhaps successful on a software and individual project level may be successful, but overall it may not lead to a successful enterprise just because of using agile software techniques.