The Collaborative International Dictionary of English v.0.48:

Tension \Ten"sion\, n. [L. tensio, from tendere, tensum, to
   stretch: cf. F. tension. See Tense, a.]
   1. The act of stretching or straining; the state of being
      stretched or strained to stiffness; the state of being
      bent strained; as, the tension of the muscles, tension of
      the larynx.
      [1913 Webster]

   2. Fig.: Extreme strain of mind or excitement of feeling;
      intense effort.
      [1913 Webster]

   3. The degree of stretching to which a wire, cord, piece of
      timber, or the like, is strained by drawing it in the
      direction of its length; strain. --Gwilt.
      [1913 Webster]

   4. (Mech.) The force by which a part is pulled when forming
      part of any system in equilibrium or in motion; as, the
      tension of a srting supporting a weight equals that
      weight.
      [1913 Webster]

   5. A device for checking the delivery of the thread in a
      sewing machine, so as to give the stitch the required
      degree of tightness.
      [1913 Webster]

   6. (Physics) Expansive force; the force with which the
      particles of a body, as a gas, tend to recede from each
      other and occupy a larger space; elastic force;
      elasticity; as, the tension of vapor; the tension of air.
      [1913 Webster]

   7. (Elec.) The quality in consequence of which an electric
      charge tends to discharge itself, as into the air by a
      spark, or to pass from a body of greater to one of less
      electrical potential. It varies as the quantity of
      electricity upon a given area.
      [1913 Webster]

   Tension brace, or Tension member (Engin.), a brace or
      member designed to resist tension, or subjected to
      tension, in a structure.

   Tension rod (Engin.), an iron rod used as a tension member
      to strengthen timber or metal framework, roofs, or the
      like.
      [1913 Webster]

WordNet (r) 3.0 (2006):

tension
    n 1: (psychology) a state of mental or emotional strain or
         suspense; "he suffered from fatigue and emotional tension";
         "stress is a vasoconstrictor" [syn: tension, tenseness,
         stress]
    2: the physical condition of being stretched or strained; "it
       places great tension on the leg muscles"; "he could feel the
       tenseness of her body" [syn: tension, tensity,
       tenseness, tautness]
    3: a balance between and interplay of opposing elements or
       tendencies (especially in art or literature); "there is a
       tension created between narrative time and movie time";
       "there is a tension between these approaches to understanding
       history"
    4: (physics) a stress that produces an elongation of an elastic
       physical body; "the direction of maximum tension moves
       asymptotically toward the direction of the shear"
    5: feelings of hostility that are not manifest; "he could sense
       her latent hostility to him"; "the diplomats' first concern
       was to reduce international tensions" [syn: latent
       hostility, tension]
    6: the action of stretching something tight; "tension holds the
       belt in the pulleys"