Which term refers to the lightness or heaviness of a substance?

The term that refers to the lightness or heaviness of a substance is best represented by "Weight." Weight is a measure of the force exerted by gravity on an object, and it directly relates to how heavy or light an object feels when lifted or supported.

In contrast, density is a physical property that relates mass to volume, indicating how much mass is contained in a given volume of a substance. While density can give an idea about the lightness or heaviness of a substance, it does not measure the gravitational force acting on that substance, which is what weight does.

Thickness typically describes the measurement of an object's size in one dimension and does not pertain to its lightness or heaviness. Lastly, mass refers to the amount of matter in an object and is often confused with weight, but it does not account for the gravitational pull acting on the object. Thus, weight is the most direct term related to the lightness or heaviness of a substance.