H

HAZMAT
Hazardous materials, as classified by the US Environmental Protection Agency (EPA). Transport of hazardous materials is strictly regulated by the US Department of Transportation.

HORSEPOWER (hp)
An obsolescent measurement of power, that is still used in the US automotive industry. The international unit is the watt (and kilowatt). One horsepower = 746W. Horsepower was based on James Watt's observations of ponies in the Eighteenth Century. In acknowledgement of his pioneering work, the basic unit of power is now the watt, whether you're discussing heat, motion, or electricity.

'Work' is movement of a specified mass through a specified distance. 'Power' is movement of a specified mass through a specified distance in a specified time, or work done over a specified time.
One horsepower is defined as 33,000 foot-pounds of work in one minute. Example: Lifting 33,000 pounds one foot in one minute, or lifting 3300 pounds ten feet in one minute.
A watt is the power required to move one kilogram through one meter in one second.

For an amusing and informative discusssion on Watt and his ponies, see: http://www.howstuffworks.com/horsepower.htm

HERTZ Hz
The hertz is the SI unit of the frequency of a periodic phenomenon. One hertz indicates that 1 cycle of the phenomenon occurs every second. For most work much higher frequencies are needed such as the kilohertz [kHz] and megahertz [MHz]. See Metric Prefixes

It is named after the German physicist Heinrich Rudolph Hertz (1857-94).
More on Hertz