From EVE History
An alt, short for alternative character, is a character that is not the main character on a account. As each EVE account has 3 character slots but only one can be trained at once, the one with most skillpoints, or the one most played with is usually considered a main.
Alternative characters are commonly used to hide the identity of a forumposter, although they have ingame uses aswell, such as scouting and price checking.
Alts can also be main or alt characters on separate accounts owned by the same person, usually this is done for nefarious reasons, or to keep ideologies, contacts, business deals, or play styles separated.
Alts originally came into fashion on the Corporation, Alliance and Organisation forum. Characters, usually from an Alliance, posted all sorts of intelligence, flame posts and flame baits regarding opposing alliances but could for various reasons not post with their main. In an attempt to reduce these usually inflamatory threads and posts, it is now prohibited to post with an Alt on this forum.