To enter the school of monism, i.e. that the 一實one great reality is universal and absolute without differentiation.

A Dictionary of Chinese Buddhist Terms, William Edward Soothill and Lewis Hodous

（術語）維摩經入不二法門品所說一實平等之理，謂之不二。明理體之無異無別也。此不二之義為法界中之一門，故名為門，通入此不二之法門，謂之入不二門。大乘義章一曰：「不二者無異之謂，即是經中一實義也，一實之理妙寂離相，如如平等，亡於彼此，故曰不二。」

丁福保《佛學大辭典》