External Information Systems Service Provider definition in Computer Security terms:
Definition(s): A provider of external information system services to an organization through a variety of consumer-producer relationships including but not limited to: joint ventures; business partnerships; outsourcing arrangements (i.e., through contracts, interagency agreements, lines of business arrangements); licensing agreements; and/or supply chain exchanges.
Source(s): NIST SP 800-161 (NIST SP 800-53 Rev. 4)
reference: CSRC Glossary