C major chord on a piano

A chord in music is when two or more notes are played together (some text books say that it is when three or more notes are played).

A chord which is built up of the 1st, 3rd and 5th notes of a scale is called a "tonic triad". If it is made from a major scale, it is a major chord. If it is made from a minor scale it is a minor chord. However, any combination of notes played simultaneously is considered a chord.