Source : Webster's Revised Unabridged Dictionary (1913)

Horse power \Horse" pow`er\
   1. The power which a horse exerts.

   2. (Mach.) A unit of power, used in stating the power
      required to drive machinery, and in estimating the
      capabilities of animals or steam engines and other prime
      movers for doing work. It is the power required for the
      performance of work at the rate of 33,000 English units of
      work per minute; hence, it is the power that must be
      exerted in lifting 33,000 pounds at the rate of one foot
      per minute, or 550 pounds at the rate of one foot per
      second, or 55 pounds at the rate of ten feet per second,
      etc.

   Note: The power of a draught horse, of average strength,
         working eight hours per day, is about four fifths of a
         standard horse power.

   {Brake horse power}, the net effective power of a prime
      mover, as a steam engine, water wheel, etc., in horse
      powers, as shown by a friction brake. See {Friction
      brake}, under {Friction}.

   {Indicated horse power}, the power exerted in the cylinder of
      an engine, stated in horse powers, estimated from the
      diameter and speed of the piston, and the mean effective
      pressure upon it as shown by an indicator. See
      {Indicator}.

   {Nominal horse power} (Steam Engine), a term still sometimes
      used in England to express certain proportions of
      cylinder, but having no value as a standard of
      measurement.

   3. A machine worked by a horse, for driving other machinery;
      a horse motor.