The Latin word libra describes a Roman unit of weight similar to a pound, and the abbreviation "lb" for the unit of weight and the sign £ (a crossed-out L) for the currency derived from this. The word "pound" itself comes from the Latin pendere, to weigh.
In the Imperial system (often referred to as the pound-inch system, or the British system in the U.S.) there are two basic pounds defined, and also an obsolete definition of one variant of the pound:
In the United States, the pound has been officially defined as a unit of mass and defined in relation to the kilogram since 1893, but its value in relation to the kilogram was altered slightly in 1894, and again to its current value in 1959 (which only differs from the 1894 definition by approximately one part in 10 million.)
There are 16 ounces in a pound (avoirdupois). The pound is equal to exactly 7000 grains, where a grain is officially defined as exactly 0.06479891 gram. The legal definition of the pound in the United Kingdom and Canada are the same as in the United States, and were unified to their current value in 1960.
A pound (troy) is a unit of mass in the U.S, Canada, and UK. The troy pound is a unit of mass equalling exactly 0.3732417216 kilograms. There are 12 troy ounces in a troy pound. A troy pound is equal to exactly 5760 grains, making 1 troy pound equal to exactly 144/175 pounds. It is part of the troy system of mass units.
They troy pound is used for measurements of precious metals such as gold, silver and platinum. Any weight measurements of precious metals always uses troy pounds and ounces, even though you will find that it is not always explicitly stated that this is the case.
One troy pound = 12 troy ounces = 240 pennyweight.
A pennyweight was literally the weight of a penny, as adopted by King Henry II (1100-1135). This was a sterling silver penny weighing 1/240th of a troy pound.