1 [Euclidean geometry] In spatial measurements, the quantity per unit area or length.
2 [physics] In a substance such as a gas, solid, or liquid, a measurement of the ratio of mass to volume.
3 [ESRI software] In ArcGIS Spatial Analyst, a function that distributes the quantity or magnitude of point or line observations over a unit of area to create a continuous rasterfor example, population per square kilometer.