A marker, found in certain positions of fixed and variable fields, that usually indicates where a valid character must be entered before the system will accept that field. Some fixed-field elements in LC authority records ( GPub, Roman) display fill characters in place of codes, indicating that those elements are not coded.
In computer terminology, a fill character is a character transmitted solely for the purpose of consuming time. It does this by filling a timeslot on a data transmission line which would otherwise be forced to be idle (empty). In this way, fill characters provide a simple way of timing required idle times.