Dictionary Definition :
Definition[1]
:
The three gates to the city of nirvāṇa, i.e. 空, 無相, and 無作 the void (or the immaterial), formlessness, and inactivity; idem 三解脫門.
Source
:
A Dictionary of Chinese Buddhist Terms, William Edward Soothill and Lewis Hodous
Definition[2]
:
(名數)三解脫門之異名。智度論二十曰:「涅槃城有三門:所謂空、無相、無作。」
Source
:
丁福保《佛學大辭典》