佛頂尊勝陀羅尼經教跡義記 2.111

2.111

唐云即大印也。解曰：所謂大印，由入毘盧遮那曼荼羅受灌頂已，從師受得本尊瑜伽三摩地五智契印，相應身心，則同即大毘盧舍那佛身及大菩薩身，能現八相成道，名大印也。

In Chinese, it means "namely, the great mudrā" (mahāmudrā). The explanation states: The so-called great mudrā means that having entered the Vairocana maṇḍala and received abhiṣeka, one receives from the master the yoga samādhi and the five-cognition mudrās of the yidam; when body and mind correspond with them, one is identically the body of the great Vairocana Buddha and the bodies of the great bodhisattvas, able to manifest the eight aspects of attaining Buddhahood, which is called the great mudrā.