Definitions
from The American Heritage® Dictionary of the English Language, 4th Edition
 n. Electricity An International System unit of power equal to one joule per second. See Table at measurement.
from Wiktionary, Creative Commons Attribution/ShareAlike License
 n. In the International System of Units, the derived unit of power; the power of a system in which one joule of energy is transferred per second. Symbol: W
from the GNU version of the Collaborative International Dictionary of English
 n. A unit of power or activity equal to 107 C.G.S. units of power, or to work done at the rate of one joule a second. An English horse power is approximately equal to 746 watts.
from The Century Dictionary and Cyclopedia
 n. The practical unit of electrical activity or power. T
from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
 n. a unit of power equal to 1 joule per second; the power dissipated by a current of 1 ampere flowing across a resistance of 1 ohm
 n. Scottish engineer and inventor whose improvements in the steam engine led to its wide use in industry (17361819)
Etymologies
Examples

(Lumens per watt is similar to miles per gallon  the higher the number represents greater efficiency)

The cost per watt is an estimate of one study likely using only two current examples from industry.
Thin Film Solar Companies Raise Hundreds of Millions in Financing  Bits Blog  NYTimes.com

Lumensperwatt is the standard used by the lighting industry to measure the conversion of electrical energy to light.
Sustainable Design Update » Blog Archive » Super Efficient Lighting

A watt is a watt, be it delivered at a potential difference of 110 volts, 220 volts, or 440 volts.

A watt is a unit of measure equal to the power produced by a current of 1 amp across the potential difference of 1 volt.

To calculate the value of Planck's constant, they are using a hightech laboratory scale known as a watt balance.
The Fate of the Kilo Weighs Heavily on the Minds of Metrologists

A number of experts argue that in twenty years pushing existing technology, solar will be less than fifty cents per installed watt, which is the cheapest energy there can be.

The watt is a common unit of electrical power, in which case one watt of power is expended when one ampere of direct current flows through a resistance of one ohm.

The product V · A is called watt in honour of the English physicist James Watt (1736  1819).

The lower manufacturing cost per watt translates into the ability to sell at a lower cost into the marketplace. "
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