A collection of services. These services communicate with each other. The communication can involve either simple data passing or it could involve two or more services coordinating some activity.
Source(s):
NIST SP 800-95
Open Grid Services Architecture Glossary of Terms (25 January 2005)
A set of principles and methodologies for designing and developing software in the form of interoperable services. These services are well-defined business functions that are built as software components (i.e., discrete pieces of code and/or data structures) that can be reused for different purposes.
Source(s):
NIST SP 800-53 Rev. 4
under Service-Oriented Architecture