1. A main tank gun operated by an experienced player who can regularly shoot down helicopters or destroy extreme distance targets with said gun.
2. A Grenade Launcher or RPG fired from a transport helicopter acheiving at least one infantry kill or two direct hits on a tank.
3. Any large projectile weapon used in situations not intended by the designer, such as firing a grenade launcher at a helicopter.
Put simply, a leetcannon is a noobcannon used in an uncommon fashion by people with skill. Uses like the above require skill, practice, and a small dose of luck. Also spelled l33tcannon or 1337cannon.
Player #1: Check this guy out, he just killed that tank form the helo!
Player #2: Holy $#!^ hitting a moving target from a moving target? No effing way
Player #3: Yes way! leetcannon pwn@g3.