1. Applying pressure to a spring, or any springy substance, thus causing it to reduce its length in the direction of the compressing force.
2. Applying pressure to a gas, thus causing a reduction in volume .
3. One of the essential factors in a internal combustion engine (fuel, air, proper proportion of mixture, compression, timing , spark ). It is the squeezing of the fuel-air mixture in the cylinder of a spark-ignition engine or the squeezing of the air in a diesel engine . Compression makes the process of combustion more effective and increases engine efficiency . Also see primary compression and secondary compression .