Plain English Meaning
Extra weight bolted into the aircraft to make it balance correctly. Once it is installed, it stays there — the pilot does not add or remove it for each flight.
Full Definition
Permanent weight installed in an aircraft at a specific location to bring its center of gravity within allowable limits or to correct an out-of-balance condition. Fixed ballast is secured to the structure and is not removed during normal operations. Its installation must be approved and recorded in the aircraft's weight and balance records.
Context Anchor
Seen in weight-and-balance discussions, aircraft records, and maintenance entries when an aircraft needs added weight to balance correctly.