Mass per unit volume. Its close counterpart is specific gravity, which is the density of the material divided by the density of purified water. The two are simply different ways of reporting the same property.
When communicating material properties between the United States and the rest of the world, citing specific gravity may be preferred over density, because it is unitless, and identical in both metric and US customary units. In metric, the conversion between density and specific gravity is trivial, because the density of water is almost exactly 1 g/cm3.
The conventional definition of density includes closed pores and voids (open pores are impractical to include, though), such that, for example, an aluminum foam has a lower density than a solid bar of the same alloy. This can also be referred to as apparent, relative, or pycnometric density. Its less common counterpart (not used in the database) is absolute or true density, which does not count pores and voids (and would therefore be the same for the above foam and solid bar).
The term bulk density refers to the density of solid but finely divided materials (granules, powder, soil, etc.), where the value depends on the degree of compaction. However, some people will use the term bulk density when referring to apparent density.
A relative of bulk density is tap density, which is the bulk density of powder or granules after the container is vibrated under a specific set of conditions. Tapping conditions are specified in standards such as ASTM B527 and ISO 3953.
While density would seem like an easy thing to measure, there is a large number of testing standards for it, targeting a variety of specialized circumstances. ASTM testing standards include B331 for sintered metals, C135 and C604 for refractories, C329 and C373 for fired whitewares, C567 for concrete, C693 and C729 for glass, and D792 and D1505 for plastics.
ISO standards include 845 for plastic and rubber foams, 1183 for plastics, 2781 for rubber, 3369 for sintered metals, 9427 for wooden panels, and 18754 for advanced ceramics.