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.
When only two notes are played together, it’s more often called “double stops” (referring to the different banks of pipes on a pipe organ, controlled by valves called “stops”).
Chords can be played on instruments such as piano, keyboard, organ, harp, guitar, harmonica, ukulele and xylophone. Chords with two notes can be played on string instruments by bowing on two strings together. By bowing on two strings and then quickly on the other two strings a violinist can sound as if he is playing a four-part chord. Some players of wind instruments are able to make chords by playing two or more harmonics together.