A waiting list was a list of people waiting for a position or item.
In 2369, Aquiel Uhnari declined Geordi La Forge's offer to find her a job as a Level 2 specialist in USS Enterprise-D's communications department, saying the waiting list was likely so long that she'd be grey before she was considered. (TNG: "Aquiel")
In 2375, Nog reasoned to O'Brien that Chief Edgar Willoughby likely did the same thing regarding requisitions. He surmised that the way to move to the top of the list was to form a relationship with the man. (DS9: "Treachery, Faith and the Great River")