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".

1. A division of electronic warfare in which actions are taken to ensure friendly, effective use of electromagnetic radiations or wavelengths in spite of an enemy's use of electronic countermeasures.
2. Efforts made to ensure effective use of electromagnetic radiation in spite of the use of countermeasures by an enemy.
3. Retaliatory tactics used to reduce the effectiveness of electronic countermeasures.

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.