Wry neck, or torticollis, is a painfully twisted and tilted neck. The top of the head generally tilts to one side while the chin tilts to the other side. This condition can be congenital (present at birth) or acquired. It can also be the result of damage to the neck muscles or blood supply. Wry neck sometimes goes away without treatment.
In the quaternions, 0 is the additive identity. In the ring of functions from R to R, the function mapping every number to 0 is the additive identity. In the additive group of vectors in Rn, the origin or zero vector is the additive identity.
Additive Identity. The additive identity is a good name for this property becuase it is a special property of addition. Ask yourself, what can I add to any integer or real number, which we'll call 'a', whose sum is 'a'. The answer is zero. Any number plus zero equals the original number.