Mirror lock-up (MLU)
Spiegelopslagvergrendeling (MLU) · Technique
A camera setting that flips the mirror up before the exposure to eliminate vibration from mirror slap.
Relevant only for DSLRs. In macro, telephoto, and long-exposure photography the mirror slap can cause noticeable blur. With MLU you flip the mirror up first, wait briefly (2 seconds), then take the photo. Mirrorless cameras don't have this issue.
Tools that help with this: