(1) The attribute of a system that allows it to operate before, during, and after an encounter with a threat. (Hitchins 2009)
(2) The manner and degree to which the tasks performed by a single software module are related to one another. (ISO/IEC/IEEE 2010)
(3) The act or state of cohering, uniting, or sticking together. (Dictionary.com 2012)
(1) Hitchins, D. 2009. "What Are The General Principles Applicable to Systems?" INCOSE Insight 12 (4) (Dec 2009): 59-63.
(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.
(3) Dictionary.com. 2012. "Cohesion." Available at http://dictionary.reference.com/browse/cohesion.
In systems engineering, cohesion refers both to the property of natural and social systems that similar elements are attracted to each other and to ability to operate in a threat environment. The context determines which definition is appropriate.