One note played alone is termed a”unison.” Two notes played simultaneously (or adjacent to one another) are termed an “interval.” Intervals are named by the distance between them, so there can be intervals of 2nds, 3rds, 4ths, 5^th, etc.  Chords are three or more notes (or pitch classes; strictly speaking, notes are the written form of pitches) played together. But these notes don’t necessarily have to be played simultaneously. Broken chords, or arpeggios, are three or more notes that aren’t played at the same time but closely enough to be heard as a group or whole. Sometimes in rock music the term “power chord” is used, meaning that the 3^rd is left out of a 3 note chord leaving just the root and 5^th.

Chords are most often named based on their number of notes or the type of intervals involved. Chords classified by note number are given names such as trichord (three notes), tetrachord (five notes), and hexachord (six notes). Chords classified by interval are given names such as tertian (third chords), secundal (second chords), and quartal (fourth chords). Sometimes chords are named based on both qualities. Tertian trichords, for example, are chords with three notes, each a third above each other. These type of chords are actually the most common in western music, found frequently in rock and pop.

These chords aren’t the only chords possible, however. There are several specialized chord types that seem to defy strict categorization. Inverted chords are created by turning any root position chord upside down. Seventh chords can be made by adding a fourth note to a triad — a third above the chord’s fifth — which makes the highest note a seventh from the root. Extended chords are those with notes that extend above a seventh, such as a ninth or an eleventh. But it’s important to mention that no extended chord can go above a thirteenth. By that point, the notes included will have already been played somewhere in the chord, taking it back down to an eleventh or thirteenth.

 A “slash chord” is a chord where the root of the chord is not the lowest note, which would include inversions, but also might include non-chordal notes. An example might be C/B.

 A “sus chord” — short for “suspension” — is a chord where the 3^rd of the chord is replaced by the 4^th.

 

The Nomenclature of Music: Unisons, Intervals, Chords from chordman on Vimeo.