(1) A set of elements in interaction. (von Bertalanffy 1968)
(2) combination of interacting elements organized to achieve one or more stated purposes (ISO/IEC/IEEE 2015)
(1) von Bertalanffy, L. 1968. General System Theory: Foundations, Development, Applications. Revised ed. New York, NY, USA: George Braziller, Inc.
(2) ISO/IEC/IEEE. 2015. Systems and Software Engineering - System Life Cycle Processes. Geneva, Switzerland: International Organization for Standardization (ISO)/International Electrotechnical Commission (IEC), Institute of Electrical and Electronics Engineers. ISO/IEC/IEEE 15288:2015. The second definition is an expanded version of the ISO/IEC/IEEE version.
Definition (1) is the System Science definition and applies to all systems: natural, social or technical.
Definition (2) is the recognized definition for systems engineers. Elements in this sense may include hardware, software, firmware, people, information, techniques, facilities, services, related natural artifacts and other support elements. This definition should be restricted to engineered systems which are created with a purpose which provides value to one or more beneficiaries.
See What is a System? for a full discussion of the nature of systems in general and the scope of engineered systems of particular interest to systems engineering.