The term madrigal can refer to two distinct forms of song:
(1) An unaccompanied song that was briefly popular in Italy during the 14th century. Texts were set to two or three free parts (“free” in that all parts were able to harmonise, instead of there being a set (usually high-voiced) melodic line with the others supplying the harmony.) The texts were nearly always secular even vernacular, usually of a light manner; pastoral, amorous, or even bawdy. It seems to have been extinct by the middle of the 15th century. Less than 200 examples survive.
(2) The term re-emerged in Italy in about 1530 to describe verse of a style and imagery that had been used by Petrarch. The seriousness of these texts and their refinement required similar properties in their musical settings. The contemporary song-type, the frottola, was not suitable.
The new form was first developed, partly by Flemish composers working in northern Italy, but also by the Italians themselves, as secular choral pieces, similar to the French chanson, but also taking some stylistic elements from the contemporary motet. During the rest of the 16th century, under the influence of the greatest composers of the age, the madrigal would grow and change and develop, until under the masterfully original hands of Carlo Gesualdo and Claudio Monteverdi in particular, the form moved out of what is generally thought of as Renaissance vocal chamber music (somewhat like a string quartet or quintet today with one voice per part) into the Baroque. Instrumental accompaniment was introduced during the first few decades of the 17th century and the form began to become indistinguishable from the cantata.
Late in the 1500s the madrigal arrived in England and forms the base for the flourishing of English song that has continued into modern times. Originally domestic music – that is, music intended to be performed in the home for the entertainment of family and friends, and for the pleasure of the performers as much as the listeners – it grew on the unparalleled richness and refinement of English poetry of that age of Shakespeare.
In its brief flowering the English madrigal developed into three distinct forms: (1) the madrigal “proper” similar to the Italian/Netherlander style, usually for 4 or more voice parts; each new verse line is introduced by a voice then taken up by the others; (2) The Ayre, where the main melody is carried by a high voice (usually the soprano) with a vocal or instrumental accompaniment; and (3) the Ballett, where the singers usually dance to the lilting-type music, and there is the classic (often mocked) "tra-la-la" refrain. All were intended for personal entertainment.
The madrigal disintegrated in Italy under the onslaught of the cantata and the rise of opera as court - and then public – entertainment. In England, the lute song usurped the role of the madrigal, although the form still holds a place in the hearts of the English – the Madrigal Society was founded in 1741 for the cultivation of madrigal singing – still meeting, it is the oldest music society in the world.
“Oxford Companion to Music”
“The Grove Concise Dictionary of Music”