Personal definition: Skill/weapon that a person relies on so heavily that their game-play or performance would be greatly diminished if they did not have it. Also can be a weapon/skill that has such an unfair advantage that you are essentially forced to use it despite whether or not you like it.
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You nailed it on the head. Year one thorn, year one felwinter, vex (pre-nerf. I'm guilty of that one.) those are just a few examples of crutch weapons.