The Collaborative International Dictionary of English v.0.48:

Tangential \Tan*gen"tial\, a. (Geom.)
   Of or pertaining to a tangent; in the direction of a tangent.
   [1913 Webster]

   Tangential force (Mech.), a force which acts on a moving
      body in the direction of a tangent to the path of the
      body, its effect being to increase or diminish the
      velocity; -- distinguished from a normal force, which acts
      at right angles to the tangent and changes the direction
      of the motion without changing the velocity.

   Tangential stress. (Engin.) See Shear, n., 3.
      [1913 Webster]

WordNet (r) 3.0 (2006):

tangential
    adj 1: of superficial relevance if any; "a digressive allusion
           to the day of the week"; "a tangential remark" [syn:
           digressive, tangential]
    2: of or relating to or acting along or in the direction of a
       tangent; "tangential forces"