Plain English Meaning
A circle drawn on the approach chart that tells you the lowest altitude you can safely fly within a set distance of a chosen reference point, with at least 1,000 feet of clearance above any obstacle in that area. It's there as a backup if something goes wrong.
Full Definition
A circular symbol on the plan view of an instrument approach chart depicting the Minimum Safe Altitude (MSA) -- the lowest altitude that provides at least 1,000 feet of obstacle clearance within a defined radius (typically 25 NM) from a designated reference point such as a navaid or fix. The circle may be divided into sectors, each showing its own minimum altitude, and is intended for use only.