Short for Military Specification.

The United States Defense Department has written specific criteria for evaluating the products it purchases.  These MilSpec''''''s often cover more extreme operating conditions (such as high temperature, low temperature, or moldy tropical climates) than most civilian products are designed to withstand.

''Somebody failed to mention being capable of withstanding 20mm direct cannon fire...'' A quality often applicable to the specifications themselves, due to sheer quantity.