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Mass density is a measure of the amount (specifically, the mass) of matter per unit volume. This is the general meaning of density (i.e., without a qualifier), but the word density is also used for other amounts "per unit volume", such as particle density (the number of particles per unit volume) and the phrase mass density is useful in contexts where another kind of density is also discussed. Such terms are commonly used for matter in space, such as portions of the interstellar medium. Some example mass densities:
| Medium | rough mass density in hydrogen atoms/meter³ |
| Earth rock | 1030 |
| liquid water | 1029 |
| Earth atmosphere at sea level | 1026 |
| typical interplanetary medium | up to 1023 |
| typical interstellar medium | 1 to 1012 |
| typical intergalactic medium | 1 to 50 |
| apparent baryonic density of the universe | 0.2-0.25 |
| critical density | 5 |
| apparent density w/dark matter etc. | same |
(This is using the mass of a hydrogen atom as a unit of mass; it is roughly a dalton, which is on the order of 1.66×10-24 grams.)