A remark set beside a thing rather than inside it. It explains without becoming part of what it explains.
Latin commentum (a devised thought, comment), from comminisci (to think up, devise), from com- (together) + the root of mens (mind); a thought set down beside a thing rather than folded into it.
A remark kept beside a thing rather than inside it, so the annotation and the thing it explains stay separately legible.