Musical harmony was invented in the late ninth century A.D. Singers in monasteries like St. Gall in Switzerland improved the basic chant by adding a voice in parallel motion singing in intervals (typically perfect fourths or fifths) along with the original melody. This was the beginning of musical harmony, and is known as organum. It later developed into counterpoint.
In A.D. 1100, an improvement known as the "florid organum" occurred, as inspired by a monastery in south-central France known as St. Martial. In the "florid organum" the original tune is sung in long noets accompanied by a voice that sings many notes for each original one, emphasizing the perfect consonances (fourths, fifths and octaves)
Groves defines a chord as: "The simultaneous sounding of two or more notes."