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"

Moby Thesaurus II by Grady Ward, 1.0:

156 Moby Thesaurus words for "tension":
   Discordia, Eris, agitation, all-overs, angst, anxiety,
   anxiety hysteria, anxiety neurosis, anxious bench, anxious concern,
   anxious seat, anxiousness, apprehension, apprehensiveness, bloat,
   bloating, breaking point, cankerworm of care, care, chill,
   chilliness, clash, clashing, clutch, coldness, concern,
   concernment, conflict, contention, coolness, crisis, crunch,
   disaccord, disaffinity, discomfort, discord, discordance,
   discordancy, disharmony, disquiet, disquietude, distension,
   distress, disturbance, dread, edginess, elongation, emergency,
   enmity, exigency, extension, extreme tension, fear, fidgetiness,
   firmness, force, foreboding, forebodingness, friction, frost, haul,
   heave, high pressure, iciness, imperativeness, incompatibility,
   incompatibleness, inflation, inharmoniousness, inharmony,
   inhospitality, inimicality, inquietude, jangle, jar, jitteriness,
   jumpiness, lengthening, malaise, mental strain, mischief,
   misgiving, nerves, nervous strain, nervous tension, nervousness,
   noncooperation, open conflict, overanxiety, overdistension,
   overdrawing, overexertion, overexpansion, overextension,
   overstrain, overstraining, overstress, overstretching, overtaxing,
   personal conflict, perturbation, pinch, pins and needles, press,
   pressure, production, prolongation, protraction, pucker, pull,
   rack, renitence, renitency, rigidity, rigidness, rigor, rub,
   snapping point, solicitude, starchiness, stew, stiffness, strain,
   strained relations, straining, stress, stress and strain,
   stressfulness, stretch, stretching, stringing out, suspense,
   swelling, tautness, taxing, tenseness, tensity, tightness,
   traction, trouble, tug, unamiability, uncordiality, unease,
   uneasiness, unfriendliness, ungeniality, unharmoniousness,
   unpleasantness, unquietness, unsociability, upset, urgency,
   vexation, worry, zeal