See also radio silence. EMCON is a military acronym, and stands for EMissions CONtrol. Essentially, it describes an operating mode where a particular unit (a ship, an aircraft, a submarine, or even a group of soldiers) acts to restrict their detectable emissions as much as possible. Given the amount of electronics used in modern war, this can be more difficult than it seems. Many radios can be detected even if they're not in active use, from leaking signals broadcast by the circuits. On submarines, emissions typically have to include acoustic signatures as well, which means machinery must be slowed or halted and even conversations inside the boat are reduced or eliminated.
Units will enter EMCON to avoid detection by the enemy. Ships under EMCON are quite difficult to find if they're not steaming strongly. Submarines generally operate under some level of EMCON all the time. EMCON can be a tactical poise as well as a general operating order; when one is being targeted by antiradiation missiles, for example, EMCON can be used to deprive those weapons of a target.