The density of a material indicates the weight of a specific volume of that material.

Density is usually defined in units of g/cm3 with water having a density of 1.0g/cm3.

Magnesium and Aluminium alloys are often known as "Light Alloys" due to their low density compared to other engineering materials.

The low density of Magnesium and Aluminium alloys make them attractive to designers for a wide range of engineering applications.

Densities of different metals and alloys vary considerably.  See below a comparison of densities of various engineering metals and alloys: