(1) an entity with discrete structure, such as an assembly or software module, within a system considered at a particular level of analysis. (ISO/IEC 1998)
(2) One of the parts that make up a system. (IEEE 2008)
(3) A set of functional services in the software, which, when implemented, represents a well-defined set of functions and is distinguishable by a unique name. (ISO/IEC 2008)
(1) ISO/IEC. 1998. Information Technology ― System and Software Integrity Levels Geneva, Switzerland: International Organization for Standardization (ISO)/International Electrotechnical Commission (IEC). ISO/IEC. 15026:1998. : 3.1
(2) IEEE. 2008. "IEEE Standard for Software and System Test Documentation". Institute of Electrical and Electronics Engineers (IEEE) Standards Association: IEEE 829-2008: 3.1.6
(3) ISO/IEC. 2008. "Information Technology ― Software and Systems Engineering ― FiSMA 1.1 Functional Size Measurement Method" Geneva, Switzerland: International Organization for Standardization (ISO)/International Electrotechnical Commission (IEC). ISO/IEC 29881:2008. A.4.
In systems terms, we use component as the generic term for the level of decomposition at which system elements are no longer considered complex, and for which specialist design disciplines can be used.