Definitions
from The American Heritage® Dictionary of the English Language, 4th Edition
 n. A space between two objects, points, or units.
 n. The amount of time between two specified instants, events, or states.
 n. One of a series of predetermined distances covered at regular time increments with intermittent periods of rest in an athletic workout.
 n. Mathematics A set of numbers consisting of all the numbers between a pair of given numbers along with either, both, or none of the endpoints.
 n. Mathematics A closed interval.
 n. Mathematics An open interval.
 n. Mathematics A halfopen interval.
 n. Mathematics A line segment representing the set of numbers in an interval.
 n. Chiefly British An intermission, as between acts of a play.
 n. Music The difference, usually expressed in the number of steps, between two pitches.
from Wiktionary, Creative Commons Attribution/ShareAlike License
 n. A distance in space.
 n. A period of time.
 n. The difference (a ratio or logarithmic measure) in pitch between two notes, often referring to those two pitches themselves (otherwise known as a dyad).
 n. A connected section of the real line which may be empty or have a length of zero.
 n. An intermission.
 n. half time, a scheduled intermission between the periods of play
 n. Either of the two breaks, at lunch and tea, between the three sessions of a day's play
from the GNU version of the Collaborative International Dictionary of English
 n. A space between things; a void space intervening between any two objects.
 n. Space of time between any two points or events
 n. A brief space of time between the recurrence of similar conditions or states
 n. Difference in pitch between any two tones.
 n. A tract of low ground between hills, or along the banks of a stream, usually alluvial land, enriched by the overflowings of the river, or by fertilizing deposits of earth from the adjacent hills. Cf. bottom, n., 7.
from The Century Dictionary and Cyclopedia
 n. A vacant or unobstructed space between points or objects; an intervening vacancy; an open reach or stretch between limits: as, the intervals between the ranks of an army.
 n. Specifically, a low level tract of land, as along a river, between hills, etc. Also intervale.
 n. Any dividing tract in space, time, or degree; an intervening space, period, or state; a separating reach or stretch of any kind: with reference either to the space itself or to the points of separation or division: as, an interval of rocky ground between meadows; to fill up an interval in. conversation with music; an interval of ease or of relapse in disease; a lucid interval in delirium; to set trees at intervals of fifty feet; to breathe only at long intervals; the clock strikes at intervals of an hour.
 n. Specifically, in entomology, one of the spaces between longitudinal striæ of the elytra. When the striæ are regular, both they and the intervals are numbered from the suture outward.
 n. In music, the difference or distance in pitch between two tones.
 n. The values given in the first column are those of the ideal intervals, such as are secured by using pure intonation; those given in the second column are those of equally tempered intonation, such as is used on keyed instruments, like the pianoforte and the organ. (See intonation and temperament.) A diatonic, interval is one that occurs between two tones of a normal major or minor scale. A chromatic interval is one that occurs between a tone of such a scale and a tone foreign to that scale. An enharmonic interval is one on an instrument of fixed intonation, that is apparent only in the notation, being in fact a unison, as, on the pianoforte, the interval from F♮ to G♭. In musical science the theory of intervals is introductory to that of chords and to harmony in general.
 n. In logic, a proposition.
 n. During or between intervals; between whiles or by turns; occasionally or alternately: as, to rest at intervals.
from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
 n. a definite length of time marked off by two instants
 n. the difference in pitch between two notes
 n. a set containing all points (or all real numbers) between two given endpoints
 n. the distance between things
Etymologies
from The American Heritage® Dictionary of the English Language, 4th Edition
from Wiktionary, Creative Commons Attribution/ShareAlike License
Examples

[2732] Strictly speaking, the interval between 11 Men and 13 Oc is fourteen days, but throughout this paper, by "_interval between_" two days, is to be understood the number of days to be counted _from_ one _to and including_ the other.

Then, assuming you still feel the total interval is nonzero, am I to presume your concept of light ray "interval" is significantly effected by traveling through a prisms or half silvered mirrors?

This certain interval will give rise to the eerie phantasmatic irreality of the Sanatorium as a result of the contamination and rapid decomposition of time.
Celebrated Animators The Quay Brothers Return with a New Feature  /Film

Little by little the tissues of reality loosen around Jozef; he becomes subject to a different clock and to the peculiar experiments with Time presided over by a mysterious Dr. Gotard [and a ventriloquizing Auctioneer]: … … here, we are always late by a certain interval of time of which we cannot define the length.
Celebrated Animators The Quay Brothers Return with a New Feature  /Film

For all we know, the 95% confidence interval is [30B, $35B,50B].

'Now, what you may or may not know about the third stellar interval is ...'

A “QT” interval is measured in seconds or in milliseconds.

Suppose the difference between using a local call centre and a Mumbai call centre for the life of the service interval is X present value dollars.
21st Century Trade Barriers, Arnold Kling  EconLog  Library of Economics and Liberty

As a result, most diesel manufacturers recommend cutting their recommended oil drain interval in half when using biodiesel fuel.

Lack of statistical significance over a given period does not mean absence of warming, it means the interval is too short for a meaningful conclusion either way.
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