1640s, noun of action from deviate (v.). Statistical sense is from 1858. Related: Deviational.
deviation de·vi·a·tion (dē'vē-ā'shən)
n.
A turning away or aside from a normal course.
An abnormality.
Deviant behavior or attitudes.
The difference, especially the absolute difference, between one number in a set and the mean of the set.