Polyphony refers to a texture in music consisting of two or more lines of melody occurring simultaneously.
Polyphony began to appear in European churches during the eleventh century. Singers began to move away from singing solely in unison or doubling in octaves and singing truly independent, different parts. The development of musical notation at this time meant that these diverging parts could be accurately recorded and recreated. This allowed for a great deal more planning and structuring by composers of music, as well as a higher level of consistency and accuracy in performance.
Monophony - music consisting of a single melody line - developed slowly to a form of polyphony called organum. In simple organum, a principal melody is doubled at the interval of a fourth or fifth below. Either or both of these voices were sometimes doubled further at intervals of an octave.
Further elaborations on organum developed, such as designating one of the lower voices a series of sustained pitches to hold as a drone while the upper voices moved along the melody proper. As the practice of writing different voices in differing rhythms became more popular, musical notation evolved to include symbols indicating approximate lengths of pitches to be sung.