An interval is the space or time between two things. When a theater performance has an interval, the audience gets a break between acts. When a doctor tells you to take medicine at regular intervals, she means you should space out the doses evenly, like every six hours.

In music, an interval is the distance in pitch between two notes. The gap between middle C and the E above it is called a major third, which describes how far apart those notes sound. Musicians practice recognizing intervals by ear, which helps them play and compose music.

In mathematics, an interval is a range of numbers between two endpoints. If your teacher says “pick a number in the interval from 1 to 10,” you can choose any number that falls between those boundaries, including the endpoints themselves (or sometimes not, depending on what kind of interval it is).

Think of intervals as the gaps or spaces that separate things, whether those things are musical notes, doses of medicine, acts of a play, or numbers on a line. When a runner trains with interval training, she alternates between sprinting hard and jogging easy, with specific time gaps between each sprint.