From the Merriam-Webster dictionary, a veteran is an old soldier of long service. A forum veteran (often just called a veteran or a vet; plural veterans or vets) is simply a member of an online community that has stuck around in that community for a long time and is usually quite knowledgeable regarding the community's members, history, etc.
Furthermore, forum veterans are often characterized as having fairly large post counts and being highly respected and/or well-known within their community (the latter depending on the size of said community).
Also, in some communities they are referred to as old-timers (especially after they refer to past events being "back in the day," etc)...
Bob is a forum veteran, because he's been a member of RaGEZONE for years, since like, forever...
Prices shown in USD.