On a person a limb is a arm or leg, and a 3d objects limb is pretty similar. Its like a part of the model that can be moved, but is joined to the model like an arm and so can not be pulled away from it, and can only rotate a certain way. Limbs joined together can form a bone structure similar to a sketeton. Obviously on humanoid models, limbs are top arms, lower arms, top and lower legs, head etc. On a car you could set the wheels as limbs, and rotate these around their one axis to spin, and around another to steer. Limbs can be added to objects, or objects can be glued to limbs, for example a character could pick up armour which would stick to him and follow his animations. Hope this is clear enogh, not quite sure how to word it
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