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.