The Collaborative International Dictionary of English v.0.48:

Power \Pow"er\, n. [OE. pouer, poer, OF. poeir, pooir, F.
   pouvoir, n. & v., fr. LL. potere, for L. posse, potesse, to
   be able, to have power. See Possible, Potent, and cf.
   Posse comitatus.]
   1. Ability to act, regarded as latent or inherent; the
      faculty of doing or performing something; capacity for
      action or performance; capability of producing an effect,
      whether physical or moral: potency; might; as, a man of
      great power; the power of capillary attraction; money
      gives power. "One next himself in power, and next in
      crime." --Milton.
      [1913 Webster]

   2. Ability, regarded as put forth or exerted; strength,
      force, or energy in action; as, the power of steam in
      moving an engine; the power of truth, or of argument, in
      producing conviction; the power of enthusiasm. "The power
      of fancy." --Shak.
      [1913 Webster]

   3. Capacity of undergoing or suffering; fitness to be acted
      upon; susceptibility; -- called also passive power; as,
      great power of endurance.
      [1913 Webster]

            Power, then, is active and passive; faculty is
            active power or capacity; capacity is passive power.
                                                  --Sir W.
                                                  Hamilton.
      [1913 Webster]

   4. The exercise of a faculty; the employment of strength; the
      exercise of any kind of control; influence; dominion;
      sway; command; government.
      [1913 Webster]

            Power is no blessing in itself but when it is
            employed to protect the innocent.     --Swift.
      [1913 Webster]

   5. The agent exercising an ability to act; an individual
      invested with authority; an institution, or government,
      which exercises control; as, the great powers of Europe;
      hence, often, a superhuman agent; a spirit; a divinity.
      "The powers of darkness." --Milton.
      [1913 Webster]

            And the powers of the heavens shall be shaken.
                                                  --Matt. xxiv.
                                                  29.
      [1913 Webster]

   6. A military or naval force; an army or navy; a great host.
      --Spenser.
      [1913 Webster]

            Never such a power . . .
            Was levied in the body of a land.     --Shak.
      [1913 Webster]

   7. A large quantity; a great number; as, a power o? good
      things. [Colloq.] --Richardson.
      [1913 Webster]

   8. (Mech.)
      (a) The rate at which mechanical energy is exerted or
          mechanical work performed, as by an engine or other
          machine, or an animal, working continuously; as, an
          engine of twenty horse power.
          [1913 Webster]

   Note: The English unit of power used most commonly is the
         horse power. See Horse power.
         [1913 Webster]
      (b) A mechanical agent; that from which useful mechanical
          energy is derived; as, water power; steam power; hand
          power, etc.
      (c) Applied force; force producing motion or pressure; as,
          the power applied at one and of a lever to lift a
          weight at the other end.
          [1913 Webster]

   Note: This use in mechanics, of power as a synonym for force,
         is improper and is becoming obsolete.
         [1913 Webster]
      (d) A machine acted upon by an animal, and serving as a
          motor to drive other machinery; as, a dog power.
          [1913 Webster]

   Note: Power is used adjectively, denoting, driven, or adapted
         to be driven, by machinery, and not actuated directly
         by the hand or foot; as, a power lathe; a power loom; a
         power press.
         [1913 Webster]

   9. (Math.) The product arising from the multiplication of a
      number into itself; as, a square is the second power, and
      a cube is third power, of a number.
      [1913 Webster]

   10. (Metaph.) Mental or moral ability to act; one of the
       faculties which are possessed by the mind or soul; as,
       the power of thinking, reasoning, judging, willing,
       fearing, hoping, etc. --I. Watts.
       [1913 Webster]

             The guiltiness of my mind, the sudden surprise of
             my powers, drove the grossness . . . into a
             received belief.                     --Shak.
       [1913 Webster]

   11. (Optics) The degree to which a lens, mirror, or any
       optical instrument, magnifies; in the telescope, and
       usually in the microscope, the number of times it
       multiplies, or augments, the apparent diameter of an
       object; sometimes, in microscopes, the number of times it
       multiplies the apparent surface.
       [1913 Webster]

   12. (Law) An authority enabling a person to dispose of an
       interest vested either in himself or in another person;
       ownership by appointment. --Wharton.
       [1913 Webster]

   13. Hence, vested authority to act in a given case; as, the
       business was referred to a committee with power.
       [1913 Webster]

   Note: Power may be predicated of inanimate agents, like the
         winds and waves, electricity and magnetism,
         gravitation, etc., or of animal and intelligent beings;
         and when predicated of these beings, it may indicate
         physical, mental, or moral ability or capacity.
         [1913 Webster]

   Mechanical powers. See under Mechanical.

   Power loom, or Power press. See Def. 8
       (d), note.

   Power of attorney. See under Attorney.

   Power of a point (relative to a given curve) (Geom.), the
      result of substituting the coordinates of any point in
      that expression which being put equal to zero forms the
      equation of the curve; as, x^2 + y^2 - 100 is the
      power of the point x, y, relative to the circle x^2 +
      y^2 - 100 = 0.
      [1913 Webster]

WordNet (r) 3.0 (2006):

power of attorney
    n 1: a legal instrument authorizing someone to act as the
         grantor's agent

Moby Thesaurus II by Grady Ward, 1.0:

67 Moby Thesaurus words for "power of attorney":
   agency, agentship, assignment, authority, authorization, brevet,
   care, change, charge, commission, commissioning, commitment,
   commutation, consignment, cure, delegated authority, delegation,
   deputation, deputyship, devolution, devolvement, displacement,
   embassy, empowerment, entrusting, entrustment, errand, exchange,
   executorship, exequatur, factorship, full power, jurisdiction,
   legation, license, lieutenancy, mandate, mission, office,
   plenipotentiary power, power to act, procuration, proxy, purview,
   quid pro quo, regency, regentship, replacement, representation,
   responsibility, subrogation, substitution, supersedence,
   superseding, supersedure, supersession, supplantation, supplanting,
   supplantment, switch, task, tit for tat, trust, trusteeship,
   vicarious authority, vicariousness, warrant



Bouvier's Law Dictionary, Revised 6th Ed (1856):

POWER OF ATTORNEY. Vide Letter of attorney, and 1 Mood. Or. Cas. 57, 58.