大方廣佛華嚴經搜玄分齊通智方軌 4.10

4.10

初釋名者，報行純熟，無相無間，故名不動地。

First, in explaining the name: the karmic practice is purely mature, signless and uninterrupted; hence it is named the Immovable Stage (Acalā-bhūmi).