法華義疏 1.265

1.265

七地得無生忍名不退，八地得無功用故名不退，故第八名不動地也。

Because on the seventh ground one attains the endurance of the non-arising of dharmas, it is called non-retrogressing; because on the eighth ground one attains effortless action, it is called non-retrogressing; hence the eighth is named the Immovable Ground.