Mass
In the real world the mass of an object is it's volume multiplied by it's density. In video games it's mostly a number to determine how many things a character can carry, or how much strength is need to move a certain object.
In the Games[edit]
Ultima Underworld[edit]
Every object in Ultima Underworld has a mass assigned to it, mesaured in the 1/10th of a stone. The Avatar's carrying capacity is displayed in stones with the decimal part truncated. Some items have zero mass, but this doesn't make them weightless when tossed around, it just means they don't count toward the carrying capacity limit.
Some items float when thrown in water and others sink and are considered lost. There doesn't seem to be a consistent system for this however, some large low mass items will sink and some small high mass items will float.
The stones in Britannia are not the same as the stone here on Earth. In real life a stone is 14 pounds or 6.35 kilograms. So that means the smallest weight in the game which is 0.1 stone would be 1.4 pounds or 635 grams. That's a big heavy for a ring on your finger. Take a look at Ultima Underworld common items and you see for example that a mage weighs 50 stone. That would be 700 pounds or 317.5 kg, and that's a pretty heavy mage. It seem more reasonable that stones in Britannia are close to kilograms here on Earth, a wirey mage weighs 50 and a stout fighter weighs 80.