collimation

n. [ Cf. F. collimation, fr. a false reading (collimare) for L. collineare to direct in a straight line; col- + linea line. Cf. Collineation. ] The act of collimating; the adjustment of the line of the sights, as the axial line of the telescope of an instrument, into its proper position relative to the other parts of the instrument. [ 1913 Webster ]

Error of collimation, the deviation of the line collimation of an astronomical instrument from the position it ought to have with respect to the axis of motion of the instrument. --
Line of collimation, the axial line of the telescope of an astronomical or geodetic instrument, or the line which passes through the optical center of the object glass and the intersection of the cross wires at its focus. [ 1913 Webster ]