Merriam-Webster Collegiate Dictionary - detent

 noun  Etymology: French detente, from Middle French destente, from destendre to slacken, from Old French, from des- de- + tendre to stretch, from Latin tendere — more at thin  Date: 1688 a device (as a catch, dog, or spring-operated ball) for positioning and holding one mechanical part in relation to another in a manner such that the device can be released by force applied to one of the parts