Plain English Meaning
A short, signed note an instructor writes in your logbook saying you have met a specific rule or are cleared to do a specific thing, such as take a test or fly solo.
Full Definition
A signed and dated statement written in a pilot or student pilot's logbook (or training record) by an authorized flight or ground instructor, certifying that the holder has met a specific requirement of the regulations — such as eligibility to take a knowledge test, take a practical test, fly solo, or operate under particular privileges.
Context Anchor
You will see this when an instructor signs you off for a knowledge test, , , certificate, or requirement.