Is it ensure, insure, or assure?
density den·si·ty (děn'sĭ-tē)
The mass per unit volume of a substance at a specified pressure and temperature.
The quantity of something per unit measure, especially per unit length, area, or volume.
A measure of the quantity of some physical property (usually mass) per unit length, area, or volume (usually volume). ◇ Mass density is a measure of the mass of a substance per unit volume. Most substances (especially gases such as air) increase in density as their pressure is increases or as their temperature decreases. ◇ Energy density is a measure of the amount of energy (often in the form of electromagnetic radiation) per unit volume in a region of space or some material. See also Boyle's law.