deviation [drill] (de-vi-a'-tion). (a) The departure of a drilled hole from its straight pathway. The hole may be either vertical or inclined and the departure may be in any direction. Deviation may be intentional, as in directional drilling, or undesirable as in crooked hole. Syn: deflection [drill]. (b) In more general use, the angle of departure of a wellbore from the vertical, without reference to direction. Cf: inclination [drill].