A tension is a note (or notes) added to a major triad, minor triad, or sixth chord which represents a logical upper extension of the triad (using intervals of a third). The following notes in a chord are considered tensions: 7, ♭7, 9, ♭9, ♯9, 11, ♯11, 13, ♭13.
These upper extension notes create "tense" musical and intervallic relationships with the pitches of the major or minor triad in the chord. The chords that contain tension notes sound somewhat more unstable than a seventh chord does. This is because the tensions "rub" against the other notes in the chord. Tensions create intervals of a richer or denser quality within the chord structure than those resulting from only the basic chord functions (root, 3rd and 5th). The seventh of a chord is often considered a chord tone, rather than a tension.
Tensions are considered non-chord tones (also called nonharmonic tones), rather than chord tones, even when they are part of a chord. Tensions enhance or enrich the sound of the chord, and modifies the basic quality of the chord sound. While a major chord with tensions still has a major quality as part of its sound, tension notes can mask the quality of the overall sound. Adding a sharp nine tension to a dominant seventh chord is an example of such a mixture.