A piece of patchwork clothing, a poem of mismatched verses, or, especially, a sort of textual collage (now more likely to be called a pastiche) assembled from the work of other authors. Typically, a cento is a poem compiled from half-lines, lines, and longer quotations from a single other work.
The cento was a Roman development of satire, and as such one of the very few forms with a Latin rather than a Greek origin. The first centos were first century parodies of Virgil; the Aeneid, as the great National Epic of the Roman Empire, became so well-known that verses from it could be recognized out of context. It made the satirical cento possible.
The Cento in Late Antiquity
Late antiquity was the cento's golden age. While it survived in the work of Ausonius as a humorous genre, it also became a favourite form of earnest Roman Christians, who were the first to employ it for serious purposes. The typical cento came to be an assembly of fragments from pagan epicists (Homer and Virgil) with a Christian meaning. Of the handful that survive, the most famous are those of a woman named Proba, who arranged 694 verses of Virgil into a re-telling of parts of the bible, and a similarly-themed Greek work (the Homerocentones) by the Empress Eudocia, wife of Theodosius II, pasted together from Homer. The surgery needed was extensive, but, as no modern person needs to be told, taking quotes out of context can go a long way towards changing a message.
The rise of the cento as a serious form was in part a result of the aridity of late antique culture. Writers of late antiquity were themselves antiquarians; they were declinists who looked back on a golden age. They admired less originality than knowledge of and affinity with the Classics. Their works were full of archaisms, written in obsolete dialects, composed in conscious imitation of the giants of the past. In the absence of artists who could or would match those of earlier periods, appropriating and remaking what they had left became natural, nearly instinctive. The column of Phocas, the Roman forum's last monument, was lifted from an earlier building; the arch of Constantine features Dacians torn from a two-century-old monument to Trajan. Like these patchwork monuments, the centos recycled treasures from a grander age. Like the mosaic they resemble, they were less technically demanding than those traditional forms.
Centos and Christians
There are also reasons the cento became a distinctively Christian form.
In the days of the persecutions, the cento could perhaps have been a way of disguising Christian content: A few favourite passages of a great pagan poet would have been perfectly innocuous, but to a converted reader the meaning of their arrangement would have been clear. This might explain both the cento's rise and its decline: By the sixth century, there was no need of such a device and the persecutions were no longer memories but merely learned history.
Then again, it has been suggested that the cento was a tool of proselytizers. Educated Romans knew Homer and Virgil intimately; the poets could be used as a bridge to prospective converts. It was a way of smuggling Christian content into pagan art, as Renaissance artists snuck pagan heroes and secular themes into art that had for a thousand years been strictly devotional. (Maybe these parallels are significant; maybe late antiquity can be thought of as the renaissance in reverse, the folding up of the classical culture that was to bloom again a thousand years later.)
More deeply, some Christians wanted to project Christianity back in time -- to give it the sanction of antiquity, even to retroactively "save" Virgil by suggesting that the Aeneid contained prophecies of the advent of Christ. The cento of Proba supports that view, and a surviving letter of St. Jerome
accuses her (and women in general) of stupidity for it.
The most important reason for the development of the form in the era of Christian predominance, however, is probably simply that it allowed literate Christians to read the great poets. Like Christian philosophy that drew on Neoplatonism or the contemporary (and closely related) biblical epics, it was -- for all that it may seem a weird hybrid to modern eyes -- a natural child of the classical-pagan and Christian-Hebrew traditions that met in the adherents of the new religion. Like others, converts to Christianity would have learned and venerated Virgil and Homer from early childhood; and though the epics were explicitly epics of the Gods and Goddesses, they would have been loth to give them up, even as many modern scientists have been willing to accommodate in their views the traditional faith in which they were brought up. With the deities excised and the meaning suitably changed, Homer became quite acceptable; and Homer in scraps was better than no Homer at all. This was the same need to find a place within the Christian world for the good parts of the pagan heritage that was to similarly preoccupy medieval intellectuals: Aquinas and others worked to reconcile Christ and Aristotle, Dante -- like Proba -- sought a place for Virgil within Christianity.
Centos and Critics
Critics have been unkind to the centos. The form doesn't make creativity impossible -- this writeup is a cento of the Oxford English Dictionary; the internet is a cento of Pi -- but it makes it difficult. Practically speaking, the ancient centos were trite and poorly fit together. They've tended to strike modern readers as pointless. And classicists who admired Homer and Virgil as much as the centonists themselves have sometimes felt pained at seeing them cut into pieces and reassembled like a game of fridge magnet poetry.
Centos and Moderns
The cento declined again after the sixth century, but it continued to reappear periodically into modern times. Under other names, the form arose again in the twentieth century; its appeal to modernists and postmodernists needs no analysis. It is the closest ancient relative of the collage, and it has analogues in other modern genres: In pastiche, in appropriated art, in "readymades", in audio collage.
Other sources (incomplete):
Proba and her cento: http://www.tl.infi.net/~ddisse/proba.html
A modern full cento: http://home.jps.net/~nada/alcalay.htm
Letters of St. Jerome: http://members.xoom.virgilio.it/blasius2/letter2/lett2lcab.html
Rosencrantz and Guildenstern Are Dead is a partial cento.
Many of Everything2's factual nodes are centos (of
Catholic Encyclopedia or Britannica articles, CDC or WHO fact sheets, the CIA World Factbook, Bartleby's sites, offline textbooks, etc.)
Principally: Half-remembered classics lectures and books, research papers, who knows what else.
The Oxford Classical Dictionary, Third Edition (1989)
Averil Cameron, The Mediterranean World in Late Antiquity
Peter Brown, The World of Late Antiquity