I know that originally a pound was a measurement found by a certain number of grains of rice. Much like the length of a foot being the length of the sitting king this is an absurd way to determine it. Who counted them? I heard that there are about 29000 grains in a pound but I am not sure about that figure.
It is a measurement of mass, although often the term is also used as a unit of weight. There have been different versions of it in history - see more at the attached link. A pound is now defined as the mass equivalent to 0.45359237 kilograms.
A pound is also a unit of currency, in the United Kingdom and some Commonwealth countries.
A pound can be an enclosed space, such as a dog pound.