A chord is a sound with a minimal of three notes, although it sounds as it were a simultaneous one,(you always hit one string as the first one)-there's no need to play it always like that. However, a real simultaneous chord can be played on such an instrument as the piano.

For example there are also broken chords(arpeggios) which is also considered as a chord. These arpeggios are played in sequence, one note after the other(ascending or descending). 

The smallest chord is based on 3 notes:a triad. I will explain this in the next chapters about the use and function of this.