apostasy / turning back (from faith)
Feminine noun (maṣdar). Verb: اِرْتَدَّ (Form VIII, to turn back/apostatize). مُرْتَدٌّ (apostate). The Quranic consequence is nullification of deeds and loss in the afterlife.
The Quran addresses riddah as a spiritual consequence, not prescribing earthly punishment - the nullification is of deeds in the afterlife.