The sounding of three or more notes simultaneously and more or less obeying the rules of Harmony.
The notes need not be on the same instrument. Chords can be formed by different instruments or voices sounding different notes, or by a single instrument with the capability of playing more than one note simultaneously. Stringed and Keyboard Instruments have this ability, wind instruments do not in general, although the reed instruments (Melodeon, Concertina, Accordian) either have buttons which sound chords or allow for more than one button to be pressed and sounded. Bagpipes such as the Uilleann Pipes can produce limited chords on keyed regulators.
A chord is three or more notes played in unison. Two notes played together form a Diad, three notes form a Triad. Where two notes played together sound harmonious, they are in accord, otherwise they are in Discord.
Number notes in an Octave from 1 to 8 (e.g. G-1, A-2, B-3, .... F#-7,g-8).
The chord is always named by it's '1' note.
If the major chord has the sixth or seventh note added, it form a chord of four notes - in this case G6 and G7.