Enterprise Architecture is generally defined in terms of its constituent architectures, namely:
Application/software architecture (see the Resources for Software Architects site)
Technology/infrastructure architecture (see the TOGAF site)
A fairly general definition of architecture in the system space (versus civil or building architectures), is:
Architecture is the high-level definition of the structure of a system, which is comprised of parts, their interrelationships, and externally visible properties.
With this definition in mind, it is all the more obvious that Enterprise Architecture is more than the collection of the constituent architectures (Business, Application, Technology, and Information). The interrelationships among these architectures, and their joint properties, are essential to the Enterprise Architecture. That is to say, these architectures should not be approached in isolation. Together, they are intended to address important Enterprise-wide concerns, such as:
meeting stakeholder needs
aligning IT with the business
seamless integration and data sharing
security and dependability
data integrity, consistency
Treating the Enterprise as a system, means taking the interactions among the constituent architectures into account. By the same token, the whole point of breaking a system into parts is so that task is less overwhelmingly complex, and specialists can focus on the parts and make progress.
Summary of Enterprise Architecture and related definitions
Malan, Ruth and Dana Bredemeyer, What is Software Architecture? at http://www.bredemeyer.com
McAfee, David, What is Enterprise-Wide IT Architecture (EWITA)? http://www.lanset.com/dmcafee/earlywork/what.htm
McAfee, David, Why EA? http://www.lanset.com/dmcafee/POVs/DMC POV.html