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.
An identity is a special kind of number. When you use an operation to combine an identity with another number, that number stays the same. Zero is called the additive identity, because adding zero to a number will not change it: the number stays the same. 0 + a = a = a + 0.