1. A quality that applies to materiel or systems: (a) possessing like and interchangeable characteristics enabling each to be utilized, or operated and maintained by personnel trained on the others without additional specialized training; (b) having interchangeable repair parts and/or components; (c) applying to consumable items interchangeably equivalent without adjustment.
2. Pertaining to equipment or systems that have the quality of one entity possessing like and interchangeable parts with another equipment or system entity.
3. Pertaining to system design in which a given part can be used in more than one place in the system, i.e., subsystems and components have parts in common.
Note: Examples of commonality include the use of a firing pin that fits in many different weapons and the use of a light source that fits in many different types of fiber optic transmitters.
Source: from Federal Standard 1037C and from MIL-STD-188
The term is also used to describe the similarities between different models of an Airliner family. For example, the commonality between the Boeing 757 and Boeing 767 aircraft allowed the hijackers participating in the September 11, 2001 Terrorist Attack to operate both the 757 and 767 with experience of only a single cockpit type.