(1) Two or more activities occurring within the same interval of time, achieved either by interleaving the activities or by simultaneous execution. (ISO/IEC/IEEE 2010)
(2) A problem, process, system, or application in which many activities are happening in parallel, the order of incoming events is not usually predictable, and events often overlap. (ISO/IEC/IEEE 2010)
(1) and (2) 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 'concurrent'.