The Collaborative International Dictionary of English v.0.48:
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,
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
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
3. A machine worked by a horse, for driving other machinery;
a horse motor.