The density, or more precisely, the volumetric mass density, of a substance is its mass per unit volume. The symbol most often used for density is "ρ" (the lower case Greek letter rho), although the Latin letter "D" can also be used. Mathematically, density is defined as mass divided by volume: 

where "ρ" is the density, "m" is the mass, and "V" is the volume. In some cases (for instance, in the United States oil and gas industry), density is loosely defined as its weight per unit volume, although this is scientifically inaccurate – this quantity is more specifically called specific weight. 

For a pure substance the density has the same numerical value as its mass concentration. Different materials usually have different densities, and density may be relevant to buoyancy, purity and packaging. Osmium and iridium are the densest known elements at standard conditions for temperature and pressure but certain chemical compounds may be denser. 

To simplify comparisons of density across different systems of units, it is sometimes replaced by the dimensionless quantity "relative density" or "specific gravity", i.e. the ratio of the density of the material to that of a standard material, usually water. Thus a relative density less than one means that the substance floats in water.

1. Are they densities of everything the same?
2. What is the densest elements at regular conditions?
3. What is density of a substance?
4. How is defined mathematically?
5. What does V stand for?
6. What about P?
7. And m?
8. Is the p used actually the English one?
9. What language is it?
10. Which letter?
11. What is another symbol for density?
12. Who gives weight per unit volume as the definition?
13. Is this a good definition?
14. Would you use it in science?
15. What would be a better term for that definition?
16. Where is density the same value as it's mass concentration?
17. To make comparisons easier what stands in for density?
18. What's an example of that?
19. What's another one?
20. What is the relative density of something that floats?

Numbered answers:
1. no
2. Osmium and iridium
3. its mass per unit volume
4. mass divided by volume
5. volume
6. density
7. mass,
8. no
9. Greek
10. rho
11. D
12. the United States oil and gas industry
13. no
14. no
15. specific weight
16. For a pure substance
17. A dimensionless quantity
18. relative density
19. specific gravity
20. less than one