Some see SOA along technical lines; others from a business perspective. Malte Poppensieker at the University of Trier pondered the true meaning of service-oriented architecture, and I think really captured its essence in this definition:
“In Service-Oriented Architecture autonomous, loosely-coupled and coarse-grained services with well-defined interfaces provide business functionality and can be discovered and accessed through a supportive infrastructure. This allows internal and external system integration as well as the flexible reuse of application logic through the composition of services."
Malte hit upon some key words here -- especially in citing "loosely coupled" and "reuse" as elements of SOA. I would add just a few words at the end:
"...to support an end-to-end business process."