The pound (abbreviations: lb or, sometimes in the United States, #) is a unit of mass (called 'weight' in everyday parlance) in a number of different systems, including English units, Imperial units, and United States customary units. Its size can vary from system to system. The most commonly used pound today is the international avoirdupois pound.
The distinction between mass and weight (or force), and its development, is discussed in the article on weight. In some circumstances, the pound is used as the name of a unit of force. That usage is discussed in the article on pound-force (a unit of force based on a weight of one (avoirdupois) pound and the acceleration of gravity at the surface of the Earth).