Glass Cannon is a term used to describe a career/class or archetype which deals heavy damage, but is very fragile (lightly armored), and typically has long ability use times or downtime between fights (like reloading a cannon). Warhammer's best examples of a glass cannon are the Bright Wizard and Sorceress careers. Both of these careers deal large amounts of damage, are very fragile, and typically have to stop and regenerate the health they have lost to Combustion or Dark Magic between fights. Due to their high damage and low survivability, a glass cannon should generally stay back from the fray and be careful of their hatred levels.
The exact origin of the term is uncertain, but it has been around since at least the 1997 'BattleNet' era.