A humorous term that denotes all characters without highly protective armor, especially caster professions. These classes are "soft" targets and therefore susceptible to damage. The antonym of a squishy is a "tank".
Additionally, this can refer to a less than desirable PvE encounter (or "hit"), where AI foes slip past the well-defended tank characters and attack the soft caster targets, resulting in a "Squishy Hit" as opposed to a "Good Hit" where the foes "Lock" onto the tanks and can be held. "Squishy hits" are a more common occurrence, and more dangerous, in higher level areas such as The Underworld and the Fissure of Woe or in Hard Mode.