Pelagianism
Pelagianism is the belief that humans are not predisposed to evil,
"The statement that all sinned in Adam, was not made because of the sin which is derived from one's birth, but because of imitation of him."
Pelagius, from c.410 CE, reported by Augustine of Hippo in Nature and Grace, 415 CE.
Antipelagianism is the belief that humans are predisposed to evil.
"We could not, without grace ... fulfil the divine commands."
Council of Carthage, 5, 418 CE. This is the clearest contemporary Antipelagian statement I have found. There may be something more pithy in Augustine of Hippo, though his Antipelagianism is often submerged in his Sacramentalism, so that he confuses matters by focussing on baptism. (It has been said that the subsequent history of "the church" has consisted of a struggle between these two elements of Augustinian thought.)
Pelagianism, the earliest and most thoroughgoing form of Bonhominalism, denies man's defection altogether. It holds that humans have no need of God to realign them.
Antipelagianism holds that humans need God to realign them.