Horsepower (hp) is a unit of measurement of power (the rate at which work is done). There are many different standards and types of horsepower. Two common definitions being used today are the mechanical horsepower (or imperial horsepower), which is 745.7 watts, and the metric horsepower, which is approximately 735.5 watts. 

The term was adopted in the late 18th century by Scottish engineer James Watt to compare the output of steam engines with the power of draft horses. It was later expanded to include the output power of other types of piston engines, as well as turbines, electric motors and other machinery. The definition of the unit varied among geographical regions. Most countries now use the SI unit "watt" for measurement of power. With the implementation of the EU Directive 80/181/EEC on January 1, 2010, the use of horsepower in the EU is permitted only as a supplementary unit. 

The development of the steam engine provided a reason to compare the output of horses with that of the engines that could replace them. In 1702, Thomas Savery wrote in "The Miner's Friend": 

So that an engine which will raise as much water as two horses, working together at one time in such a work, can do, and for which there must be constantly kept ten or twelve horses for doing the same. Then I say, such an engine may be made large enough to do the work required in employing eight, ten, fifteen, or twenty horses to be constantly maintained and kept for doing such a work…
What does hp stand for?
Is there only one type of Hp?
What is one type?
What else is that type called?
Is that 735 watts?
How many watts is it?
Which type is approximately 735 watts?
Who is a Watt named for?
When did this occur?
What did the term compare the output of steam engines to?
Did it ever include engines of other types?
What is one kind?
Was the meaning the same no matter where you were?
What is it known as now for the most part?
What happened in January of 2010?
What did that mean for the EU?
Who did something in 1702?
What did he do?
Did he think an engine could do the work of twenty horses?
How would it have to be maintained?
Horsepower
Horsepower
No
Two common definitions
imperial horsepower
 imperial horsepower
mechanical horsepower
mechanical horsepower
no
745.7
745.7 watts
745.7 watts
metric horsepower
 metric horsepower, which is approximately 735.5 watts
James Watt
James Watt
late 18th century
 late 18th century 
The power of draft horses.
 the power of draft horses
Yes.
 other types of piston engines, as well as turbines, electric motors and other machinery.
piston engines
 piston engines
No.
 Most countries now use the SI unit "watt" f
SI unit .
SI unit 
EU Directive 80/181/EEC
 EU Directive 80/181/EEC on January 1, 2010
That the use of horsepower in the EU is permitted only as a supplementary unit.
the use of horsepower in the EU is permitted only as a supplementary unit
Thomas Savery
Thomas Savery wrote in "The Miner's Friend": 
Wrote in "The Miner's Friend":
 wrote in "The Miner's Friend": 

Yes.
such an engine may be made large enough to do the work required in employing eight, ten, fifteen, or twenty horses to be constantly maintained and kept for doing such a work…
Constantly.
constantly maintained