Density and dense usually refer to a measure of how much of some entity is within a fixed amount of space. Types of density include: Physics Density of mass Density, mass per unit volume Bulk density, mass of a particulate solid or powder divided by the volume it occupies Particle density (packed density) or true density, density of the particles that make up a particulate solid or a powder Neutral density, mass density of seawater Area density or surface density, mass over a (two-dimensional) area Linear density, mass over a (one-dimensional) line Relative density or specific gravity, a measure of density in comparison to the density of something else Vapour density, a relative density used for gases Densities of other entities Number density, number of particles per unit volume, area, or length Current density, the ratio of electric current to area Charge density, the electric charge per volume Energy density, potential energy per unit volume or mass, depending on context Force density, force per unit volume Optical density, the absorbance of a material Densitometry, measuring the optical density of photographic paper, photographic film and x-ray film Particle density (particle count), number of particles per unit volume Power density, power per unit volume Surface power density, power per unit area Mathematics Probability and measure theory Probability density function, a function which maps probabilities across the real line and whose integral is 1 Density estimation is the construction of an estimate of a probability density function Kernel density estimation, used in statistics to estimate a probability density function of a random variable Lebesgue's density theorem Number theory Schnirelmann density Natural density (also called asymptotic density) Dirichlet density Geometry Packing density Density (polytope) Density on a manifold Tensor density in differential geometry Topology Dense set and nowhere dense set Dense-in-itself is a set that contains no isolated points Other fields Density (graph theory), the fraction of possible edges that exist in a graph Dense order in order theory A coinitial subset in the mathematical theory of forcing, in set theory Dense submodule in abstract algebra Other scientific fields Population density, population per unit land area Urban density, in urban planning and urban design Areal density (computer storage), bits (how computers store information) over an amount of area or volume Nutrient density, the proportion of any array of a single nutrient or nutritional factor, or of numerous nutrients in foods, often ordered by different