The spacing in pitch or frequency between two sounds; the frequency interval is the ratio of the frequencies or the logarithm of this ratio.

A set of numbers which consists of those numbers that are greater than one fixed number and less than another, and that may also include one or both of the end numbers.

The time separating two events, or the distance between two objects.

1. In special relativity, the Lorentz invariant quantity c²(Δt )²- (Δx )²-(Δy )²-(Δz )², where c is the speed of light, Δt is the difference in the time coordinates of two specified events, and Δx, Δy, and Δz are differences in their x, y, and z coordinates, respectively.

2. In general relativity, a generalization of this concept, namely the sum over the indices μ and ν of g_μνdx^μdx^ν, where dx^μ and dx^ν are the differences in the x^μ and x^ν coordinates of two specified neighboring events, and g_μν is an element of the metric tensor.