Density

Density is mass per unit volume, $\rho = m/V$, and the AP Physics 1 equation sheet prints it in exactly that form. The SI unit is $\text{kg/m}^3$: fresh water is about $1000\ \text{kg/m}^3$, and air near sea level is close to $1.2\ \text{kg/m}^3$.

Essential knowledge 8.1.A.3 introduces it as the property that characterises a fluid, and the CED pairs Unit 8 with a warning about vocabulary: mass, volume, weight, size and density are five different words and the exam expects you to use them apart.

The reason density earns a unit of its own is the float test. An object released in a fluid floats if its average density is below the fluid's and sinks if it is above, no matter what the two things weigh. A steel hull floats because the air it encloses drags its average density under that of water. A freely floating object settles with the fraction $\rho_{\text{object}}/\rho_{\text{fluid}}$ of its volume under the surface, which is where the familiar iceberg figure comes from.

The conversion is a standing trap. Water quoted as $1\ \text{g/cm}^3$ is $1000\ \text{kg/m}^3$, because one cubic centimetre is $10^{-6}$ of a cubic metre, not $10^{-2}$.

For the float-or-sink reasoning worked through with numbers, see Fluids in AP Physics 1.