The spreading of a cathode-ray stream due to repulsion of like charges (electrons).

The ratio of the area of any section of fluid emerging from a nozzle to the area of the throat of the nozzle.

For a vector-valued function, the sum of the diagonal entries of the Jacobian matrix; it is the scalar product of the del operator and the vector.

The two-dimensional horizontal divergence of the velocity field.

In a nuclear reactor, the condition wherein the number of neutrons produced increases in each succeeding generation.

A horizontal flow of water, in different directions, from a common center or zone.

The spreading apart of a beam of particles of light.