Plain English Meaning
It's an extra rating added onto a pilot or instructor certificate someone already holds, giving them the legal authority to fly or teach in gliders.
Full Definition
An additional category rating added to an existing pilot or flight instructor certificate that authorizes the holder to act as pilot or instructor in gliders. The applicant must meet the FAA's aeronautical knowledge, flight proficiency, and experience requirements specific to gliders, and the rating is issued by endorsement and practical test rather than by issuing a new certificate.
Context Anchor
Seen in endorsement records, training plans, and practical test preparation when a pilot or instructor is adding glider privileges to an existing certificate.