1.nounA measure of the mass of matter contained by a unit volume.
2.nounThe ratio of one quantity, representing something of interest, to another quantity representing space, area, or extent in which the thing of interest is distributed.
"The number of particles per unit volume of a specified volume can be considered to be the particle density for the specified volume."
3.nounThe probability that an outcome will fall into a given range, per unit of that range; the relative likelihood of possible values of a continuous random variable.