Discrimination against a system's users based on their serially assigned UIDs. For example, a few Slashdot users have had statements to the effect of "Never trust a user over 90000" in their signatures. On ICQ, a user's UID is thought by some to be inversely proportional to the user's l33tness. The reasoning behind this is thought to be that users with high UIDs are clueless newbies.
I do not support UIDism.