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.
The inverse of an operation undoes the operation: division undoes multiplication. A number's additive inverse is another number that you can add to the original number to get the additive identity. For example, the additive inverse of 67 is -67, because 67 + -67 = 0, the additive identity.