Definitions
from The American Heritage® Dictionary of the English Language, 4th Edition
 n. A unit of energy, especially electrical energy, equal to the work done by one watt acting for one hour and equivalent to 3,600 joules.
from Wiktionary, Creative Commons Attribution/ShareAlike License
 n. A unit of electrical energy equal to the power of one watt in use for one hour; often used as a unit of electricity consumption
from The Century Dictionary and Cyclopedia
 n. The energy derived from an activity or power of one watt maintained for one hour: a practical unit of energy.
from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
 n. a unit of energy equal to the power of one watt operating for one hour
Etymologies
Sorry, no etymologies found.
Examples

It supports WiFi, Bluetooth and aGPS and includes a builtin 25watthour lithium battery.

For comparison, the iPad 2 has a 25 watthour battery; the 11in MacBook Air model has a 35Wh battery, going up to 40Wh in the 13in model.
Apple working on ARM port for Mac OS X: will Macbook Airs see it first?

In the past 10 years alone, the energy density of lithiumion batteries has doubled, and the price per watthour has fallen by more than 80%.

It is expected that for every square meter of surface, the process could generate about 1 watthour per year.

A BTU, or British thermal unit, is a unit of energy equivalent to about onethird watthour of electric power.

In the inchpounds unit system, a watthour is equivalent to 3.412 Btu.

The watthour It is not an SI unit, despite being based on the watt.

One watthour is the amount of electrical energy equivalent to a onewatt load drawing power for one hour.

The watthour (symbol W·h) is a unit of energy commonly used to measure electricity.

This may not be taken as an accurate figure, however, as the watthour capacity of a cell depends very largely, not only on the make of the cell, but on the rate of its discharge.
Cyclopedia of Telephony & Telegraphy Vol. 1 A General Reference Work on Telephony, etc. etc.
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