Also known as the waiting list, the waitlist is an official list of fans who have expressed interest in joining an apa (or, must less commonly, a club or con) with an upper membership limit after that limit has been reached.
Unlike the open-ended mundane version, many fannish apas have a limited number of members. When that number is reached and the apa is not invitational, people who wish to join are kept, in the order of their application, on a "waiting list" in the Official Organ of the apa. When a member drops out or is dropped – resignation, failure to meet activity requirements, failure to pay dues – the first person on the waiting list is invited to join. (When the apa is invitational, either members stop suggesting new members until a new opening occurs or they suggest a simultaneous increase in the size of the membership.)
People on these lists are sometimes called Waiting Listers or, at other times, WLers.
The first waitlist was created for The Cult (probably because of its very low membership limit of 13). The FAPA waitlist grew to the point where at its peak it took around five years on the waitlist to gain FAPA membership — the story was that neos would join the FAPA waitlist as soon as they heard about FAPA, and would still be on it when the gafiated.