Characterization of atoms by their nuclear properties, as the number of protons and the number of neutrons. The different nuclides of an element are its isotopes (equal proton number, but different neutron numbers). Isomers of this particular nuclide are equal in the proton and mass numbers, but differ in their energy content. Unstable nuclides which are radioactive are called radionuclides.

See also Isotope, Isomer and Decay.

(Osm) A unit of osmotic pressure used in physical chemistry, cell biology, and medicine.
Definition: 1 osmole is the osmotic pressure of a one molar solution (that is, a solution with a concentration of one mole per liter of solvent) of a substance that does not dissociate.
If chemical solutions are separated by a semipermeable membrane (a membrane that resists the passage of dissolved substances but permits the passage of the solvent, usually water), then the solvent will diffuse across the membrane to equalize the concentrations. This process is called osmosis.
Solutions with higher concentrations of dissolved substances are said to have higher osmotic pressure than solutions having lower concentrations; thus the solvent moves from an area of low osmotic pressure to an area of higher osmotic pressure.
Osmotic pressure depends on the total number of dissolved particles, so for a substance that dissociates into two ions, such as ordinary salt (sodium chloride), a one molar solution has an osmotic pressure of 2 osmoles. In practice, most measurements are in milliosmoles (mOsm). Typical values range from 20 mOsm for fresh water through 290 mOsm for typical human blood plasma to 1010 mOsm for salt water from the open ocean.
See also Part Per Million.

The energy of a photon is direct proportional to its frequency.
E = h * v
E = Energy (J joule)
h = Planck's constant, 6.626 x 10^-34 J-s (Joule-seconds)
v = Frequency (Hz)
This relation between energy and frequency of a photon is known as Planck's relation.

See also Photon.

(PSF) The point spread function describes the response of an imaging system to a point or impulse input. A hypothetical point object will generally have an extended (blurred) image resulting from the imaging process; this is the point spread function characterizing the imaging process. Considering any object as composed of an assembly of point objects, knowledge of the PSF permits the prediction of how the object will be imaged, assuming linearity of the imaging process.

See also Resolution, Phantom and Daily Quality Assurance.

Two quantities (x, y) are called proportional if they vary in such a way that one of the quantities is a constant multiple of the other, or equivalently if they have a constant ratio (k = y/x). This constant ratio (k) is called the proportionality constant or constant of proportionality of the proportionality relation.