Mirror Lock-up
A function that raises an SLR's mirror in a separate step before the shutter fires, so its vibration has time to settle.
In depth
Mirror slap transmits a distinct shock through the body that blurs detail most severely between about 1/30 and 1/2 second, precisely where a tripod would otherwise be delivering its best work. Locking the mirror up a second or two ahead removes it entirely. Mirrorless cameras have no mirror, so the equivalent concern is shutter shock, addressed by electronic first curtain instead.
Examples in practice
A 1/8 s tripod shot of a cathedral interior gains visible crispness with mirror lock-up and a two-second delay.