5 Answers

How compacted or hard a material is.

1 noun
The quality or condition of being dense.

2 noun
The quantity of something per unit measure, especially per unit length, area, or volume.

3 noun
The mass per unit volume of a substance under specified conditions of pressure and temperature.

4 noun
Computer Science The number of units of useful information contained within a linear dimension.

5 noun
The number of individuals, such as inhabitants or housing units, per unit of area.

6 noun
The degree of optical opacity of a medium or material, as of a photographic negative.

7 noun
Thickness of consistency; impenetrability.

8 noun
Complexity of structure or content.

9 noun
Stupidity; dullness.

thickness or compacted. Some people say "so and so' has dense brains.

Measures; Hardness to softness . Less air molecules is more dense as to hard woods and softer woods to balsa, more air molecules. 

The Population "density (people per sq. km)" in Belgium was 359.29 in 2010, according to a World Bank report released in 2011.