1. Something which is done in reaction to and as a defense against a hostile action by someone else, or something that is done in order to deal with a threat.
2. An action taken to offset another action.
3. Action taken to oppose, to neutralize, or to retaliate against some other action.
4. Etymology: combined with the Latin prefix contra, "opposite, contrary to, against, in return" + measure, "plan of action".

Reconnaissance by aircraft equipped with electronic devices capable of locating enemy radar stations, determining their area coverage, and making radarscope recordings for combat mission folders.

1. A division of electronic warfare in which actions are taken to interfere with a military enemy's use of the electromagnetic or a range of electromagnetic radiation or waves, used in military air defenses and interceptions.
2. The disruption of the operation of a military enemy's equipment; as by jamming radio or radar signals.
3. A military offensive or defensive tactic or device using electronic and reflecting apparatuses (apparatus or systems allowing certain functions) to reduce the military effectiveness of enemy equipment involving electromagnetic radiation; such as, guidance, radar, communication, or other radio-wave devices.