(1) An adjective describing a system's design or code that is difficult to understand because of numerous components or relationships among components (ISO/IEC/IEEE 2010).
(2) An adjective describing a system or component that has a design or implementation that is difficult to understand and verify (ISO/IEC/IEEE 2010).
ISO/IEC/IEEE. 2010. Systems and Software Engineering - System and Software Engineering Vocabulary (SEVocab). Geneva, Switzerland: International Organization for Standardization (ISO)/International Electrotechnical Commission (IEC)/ Institute of Electrical and Electronics Engineers (IEEE). ISO/IEC/IEEE 24765:2010.
Modified from the definition of complexity in the SEVOCAB.