Its story
The term 'Semi-Pelagianism' did not exist in antiquity: it was coined in the late 16th century, in the grace disputes surrounding Luis de Molina (its first known use is attributed to Theodore Beza), and was applied retrospectively to a group of 5th-century monks in southern Gaul. Their contemporaries called them 'Massilians' (after Marseille). They were not followers of Pelagius: they accepted original sin and the need for grace, and explicitly rejected Pelagian theses.
Their objection was aimed at Augustine's late works on predestination and efficacious grace. John Cassian, in his Conference 13, held that sometimes God goes before the will and sometimes the will takes the first step and God sustains it. They feared that an absolute predestination would make ascetic effort, exhortation and responsibility pointless. Vincent of Lérins and later Faustus of Riez belong to this current.
Augustine replied around 428–429 with On the Predestination of the Saints and On the Gift of Perseverance; his disciple Prosper of Aquitaine continued the defense. Almost a century later the Second Council of Orange (529), led by Caesarius of Arles, taught that Adam's sin affected all humanity and that 'even the beginning of faith' is the work of grace, while rejecting predestination to evil. Pope Boniface II approved it (531). Orange was a local synod, but its authority grew from the 16th century, when it was rediscovered.
During and after the Reformation, 'Semi-Pelagian' became a recurring charge: Lutherans and Reformed leveled it against late-medieval Catholic theologians and against Arminianism; Dominicans and Jesuits hurled it at each other in the De auxiliis controversy. Many Orthodox note that the category depends on an Augustinian framework their tradition does not share in the same way, and that Eastern 'synergy' should not be identified with it. The label is therefore best used with care.