The term Dram comes from the old English apothecary system and are used to measure units of liquid volume. One dram is equal to 1/8 of an ounce.
Today, it’s used for Whisky (particularly Scotch) and refers to a single serving of whisky. There isn’t an official measurement anymore and for most, a dram is considered to be the amount of whisky that you could reasonably swallow in one mouthful.