What is "deviation of the compass"

deviation \de`vi*a"tion\, n. [LL. deviatio: cf. F. d['e]viation.]

1. The act of deviating; a wandering from the way; variation from the common way, from an established rule, etc.; departure, as from the right course or the path of duty.

2. The state or result of having deviated; a transgression; an act of sin; an error; an offense.

3. (Com.) The voluntary and unnecessary departure of a ship from, or delay in, the regular and usual course of the specific voyage insured, thus releasing the underwriters from their responsibility.

4. (Statistics, Physics) the difference between an expected value of an observation or measurement and the actual value.

   Deviation of a falling body (Physics), that deviation from a strictly vertical line of descent which occurs in a body falling freely, in consequence of the rotation of the earth.

   Deviation of the compass, the angle which the needle of a ship's compass makes with the magnetic meridian by reason of the magnetism of the iron parts of the ship.

   Deviation of the line of the vertical, the difference between the actual direction of a plumb line and the direction it would have if the earth were a perfect ellipsoid and homogeneous, -- caused by the attraction of a mountain, or irregularities in the earth's density.