1. The state or quality of being dense; compactness; closely set or crowded condition.
2. Thickness of consistency; impenetrability.
3. Complexity of structure or content.
4. The number of inhabitants, dwellings, or the like, per unit area: "The commissioner noted that the population density of certain city blocks had fallen dramatically."
5. In physics, mass per unit volume.
6. The quantity of electricity per unit of volume at a point in space, or the quantity per unit of area at a point on a surface.
7. The degree of opacity of a substance, medium, etc., that transmits light.
8. In photography, the relative degree of opacity of an area of a negative or transparency, often expressed logarithmically.
9. With computers, a measure of the compactness of data stored on an external storage medium, as disk or tape, or displayed on a CRT or other screen; also, the number of units of useful information contained within a linear dimension.
10. Thickness of consistency; impenetrability.

In meteorology, the ratio of the mass of a portion of the atmosphere to the volume it occupies: When the couple went out for a walk in the early morning, they could hardly see in front of them due to thick fog, or an atmospheric density.

The current per unit of a cross-sectional area of a conductor.

The electric field intensity multiplied by the permittivity (measure of the ability of a nonconducting material to retain electric energy when placed in an electric field) or the property of a dielectric medium which determines the forces that electric charges placed in the medium exert on each other.

1. The number of electrons per a unit volume or in a unit volume.
2. The quantum mechanical probability density for an electron.

The vector sum of the spin angular momenta of electrons at each point in a substance per unit volume.

1. A contoured representation of electron density at various points in a crystal structure expressed in electrons per cubic angstrom with the highest near atomic centers.

The map is calculated using a Fourier synthesis, a summation of waves of known phase, frequency, and amplitude.

2. A three-dimensional representation of the electron density of a molecular structure based on x-ray diffraction data.

1. The ratio of the average amount of current density at a given opening to the average current density at the cathode surface.
2. A ratio of the average current density at any specified opening through which the electron stream passes to the average current density at the cathode surface.

1. The amount of a given type of radiation that crosses a specified area within a specified period; such as, the number of photons passing through one square centimeter of a target in one second.
2. The electric flux passing through a surface, divided by the area of the surface.
3. A vector field that represents the differential flux of field lines per unit area.

A process by which radiation flux density is tracked within a reactor or other radiation source.

The number of gamma rays passing through a unit area in a unit of time.

1. In atmospheric electricity, the number of ions per unit volume of a given sample of air; more particularly, the number of ions of given type (positive small ion, negative small ion, positive large ion, etc.) per unit volume of air.
2. The number of ions per unit volume.
3. The density of ions in a gas.

1. A radiometric term for the rate at which radiant energy is transferred across a unit area of a surface, commonly measured in watts per square meter.
2. A radiometric term for the rate at which radiant energy in a radiation field is transferred across a unit area of a surface (real or imaginary) in a hemisphere of directions.

In general, irradiance depends on the orientation of a surface. The radiant energy may be confined to a narrow range of frequencies (spectral or monochromatic irradiance which is characterized by a single frequency) or integrated over a broad range of frequencies.

1. An optical filter that reduces or diminishes the intensity of transmitted light without significanly changing its color.

It is used on a camera to decrease the amount of light entering a camera when the lens cannot be stopped down sufficiently for use with a given film.

2. A light-absorbing filter whose absorption spectrum is moderately flat.

Depending on the type, the absorption curve is flat primarily in the visible spectral range, or it may extend to varying degrees beyond the visible range.

For video microscopy, this is an important procedure since the absorbance may or may not extend into the near-infrared region where the sensitivity of many video image pickup devices is very high.

The density of a liquid and of a saturated vapor with which it is at equilibrium at a given temperature.

The amount of energy per unit in an electromagnetic field: Power density is normally stated in units of watts per square centimeter.

A measure of proton concentration, or the number of atomic nuclei for given volumes.

It is one of the major determinants of magnetic resonance signal strength in hydrogen imaging.

Another term for absolute humidity; that is, the mass of moisture in relation to the unit volume of space that it occupies.