Plain English Meaning
A small list on the controller's screen that holds aircraft the radar has briefly lost track of, so their information isn't lost while the system waits to pick them up again.
Full Definition
A list shown on the ARTS III / DBRITE radar display that contains aircraft data tags the system is temporarily holding because radar is no longer receiving fresh position updates for them. The system 'coasts' a track by continuing to display its last known data for a short period, and 'suspends' it when updates remain missing, moving the tag off the active radar picture and into this side list until contact is reestablished or the track is dropped.
Context Anchor
Seen in descriptions of older air traffic control radar displays, especially the ARTS III/DBRITE display shown in the .