in·ter·val 
(ĭntər-vəl)
(ĭntər-vəl)n.
Idiom: 1. A space between objects, points, or units, especially when making uniform amounts of separation: We set up hurdles at intervals of 15 yards around the track.
2. An amount of time between events, especially of uniform duration separating events in a series: We ran laps at 30-second intervals.
3. A segment of an athletic workout in which an athlete runs, swims, or does other exercise over a series of predetermined distances at regular time increments with intermittent rests.
4. Mathematics
a. A set of numbers consisting of all the numbers between a pair of given numbers along with either, both, or none of the endpoints.
b. A closed interval.
c. An open interval.
d. A half-open interval.
e. A line segment representing the set of numbers in an interval.
5. Chiefly British An intermission, as between acts of a play.
6. Music The difference, usually expressed in the number of steps, between two pitches.
at intervals
In a series separated by space or time: trees planted at intervals; coughing at intervals.
[Middle English intervalle, from Old French, from Latin intervallum : inter-, inter- + vallum, rampart.]
in′ter·valic, in′ter·vallic (-vălĭk) adj.