Commercial-off-the-shelf (COTS) parts and equipment are those items that can be built into a system without being custom-built to meet military specifications, project requirements that were created without first checking if a commodity part in the general marketplace would be adequate, or that can be bought without extensive paperwork. One reason that military and government systems can be so expensive is that relatively small volumes of custom-made components have to be manufactured, with all the attendant tooling and assembly line changes, new documentation, and administrative handling.
COTS does not mean unreliable or unable to work under many extreme conditions. Certain COTS components, for example, meet the rigid specifications and testing of medical application where lives depend on them. With the commercialization of space, parts meant to operate in vacuum and in the expected temperature extremes are in regular commercial catalogs. These will be more expensive than parts designed for home use, but still less costly than completely custom items.
A drive to "buy COTS" can be overdone, because some environments truly do have special requirements. "Horror stories" of $600 toilet seats and $5000 coffeemakers often are heard as examples of military waste. In the case of the toilet seat, a COTS one would have been adequate; the cost came from massive administrative overhead because the buyer required formal bidding and an extensively documented proposal. When a coffeemaker has to be safe in an antisubmarine aircraft that may be in extreme weather, with the pot firmly restrained and the heater running on aviation-standard rather than commercial-standard electrical power, custom engineering indeed was necessary.