The term belcanto (beautiful singing), has probably meant different things at different periods in music history, but its origins are shrouded in the past.
The vocal works of Mozart, as well as such Italian composers as Donizetti, Bellini, Rossini and the early works of Verdi, are considered to fall under the rubric of belcanto.
These composers crafted vocal works with long, often florid, phrases that showcased the singer's vocal prowess and pyrotechnic capabilities.
Both long, sustained legato phrases (sostenuto) and rapid passages requiring agility (fioritura) characterize the bel canto vocal line. Thus, beautiful vocal lines were characteristic of this music; the other half of the bel canto equation is that the art of vocal production or vocal technique had been elevated to such a high level of skill and refinement, that bel canto was actually a marriage of consummate vocal technique and the beauty of composition.
So both the singing itself and the music written for the voice were "bellissimi." The Italian school of singing has produced singers with a characteristic noble posture which encourages proper breathing to produce the most beautiful sound—so beautiful, in fact, that the Italian school of singing is nowadays the general international standard for opera superstars.