(1) Anything affecting a subject system or affected by a subject system through interactions with it, or anything sharing an interpretation of interactions with a subject system. (IEEE 1175.1-2002 (R2007), 3.6)
(2) The surroundings (natural or man‐made) in which the system‐of-interest is utilized and supported; or in which the system is being developed, produced or retired. (INCOSE 2010)
(1) IEEE. 2002. IEEE Guide for CASE Tool Interconnections - Classification and Description, 1175.1-2002.
(2) INCOSE. 2010. INCOSE Systems Engineering Handbook, version 3.2. San Diego, CA, USA: International Council on Systems Engineering (INCOSE), INCOSE-TP-2003-002-03.2.
(1) is a system science definition and can be applied to any system.
(2) is an engineered system definition, and distinguishes between the different environments that exist during the life of a system.