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"

Moby Thesaurus II by Grady Ward, 1.0:

34 Moby Thesaurus words for "tangential":
   approaching, asymptotic, brushing, centripetal, centrolineal,
   concurrent, confluent, confocal, connivent, contacting, contingent,
   converging, digressive, divergent, extraneous, focal, glancing,
   grazing, impingent, impinging, in contact, irrelevant, meeting,
   mutually approaching, nudging, osculatory, peripheral, radial,
   radiating, rubbing, tangent, touching, uniting, unrelated