n. The act of deviating; a turning aside from the way or course.n. Departure from a certain standard or from a rule of conduct, an original plan, etc.; variation; specifically, obliquity of conduct.n. In com., the voluntary departure of a ship without necessity, or without reasonable cause, from the regular and usual course of the specific voyage insured.n. In astronomy, the oscillatory motion of a plane; especially, in the Ptolemaic system, the oscillation of the plane of the orbit of a planet, which was supposed to account for certain inequalities in the latitude.n. If the law of error be stated by the equation , more conveniently written , then is called by Pearson the standard deviation. The square root of the average of the squared departures from the mean in both directions is a measure of variability called the standard deviation.