The two doors of ingress and egress, i.e. enter the gate of self-purification and adornment, then go forth 出 to benefit and save others.

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

（名數）入門與出門之稱。入門者，謂修淨業而入於極樂功德莊嚴中之自利門也。出門者，謂以慈悲心為苦惱眾生出施教化之利他門也。淨土論所說之五念門與五功德門中，前四門為入門，後一門為出門也。

丁福保《佛學大辭典》