What is an interval?

 
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In music theory, an interval is a combination of two notes, or the ratio between their frequencies. Two-note combinations are also called dyads. Although chords are often defined as sets of three or more notes, intervals are sometimes considered to be the simplest kind of chord.
Intervals may be described as:
Horizontal, linear, or melodic if they sound successively.
Vertical or harmonic, if the two notes sound simultaneously
In Western culture, the most common method to classify and name intervals is based on their quality (perfect, major, minor, etc.) and number (unison, second, third, etc.). For instance, two frequently used types of interval are called minor third and major third.
Minute intervals (commas, and microtones) can be formed by the notes of musical scales containing more than 12 pitches (e.g., by the notes A♭ and G♯ found in some extended scales), or by two notes of the same name, but tuned differently (e.g., the syntonic comma is sometimes defined as the difference between an F♯ tuned using the D♭ased Pythagorean system, and another F♯ tuned using the D♭ased quarter-comma meantone system). The difference in pitch can be so small that it cannot be perceived.

http://en.wikipedia.org/wiki/Musical_interval