A troy weight containing twenty-four grains, or the twentieth part of a troy ounce; 1.555 grams; as, a pennyweight of gold or of arsenic. It is abbreviated dwt or pwt. It was anciently the weight of a silver penny, whence the name.
a penny weight is a means of weighing precious metals. Comes from the British weight system. There are 20 pennyweights in a troy ounce. Click here for more weight comparisions.
A measurement of weight, 1 pennyweight = 24 grains, 20 pennyweight = 1 troy ounce