阿吽

Aun means the unity of all things. The first and last thing. The Universe. The beginning and end of all things.

It means everything entering Harmony, Synchronization.

The origin of Aun is Sanskrit a-hūm.

阿 - "a" is an open syllable, pronounced with an open mouth, representing exhaling.

吽 - "un" is a closed syllable, pronounced with a closed mouth, representing inhaling.

Aun as such therefore also represents breath, breathing as such. For Sumou it specifically signifies Aun Breath.

Aun is also reflected in Aun no chikarawaza, one of Sumou Sayings.

At temples and shrines where guardian Ni Ou and Koma Inu stand at the gates in pairs, they also symbolize Aun. One will have an open mouth, symbolizing the Beginning, uttering "a", and the other will have a closed mouth, symbolizing the End, uttering "un".

Aun was a Buddhist concept originally and Aun Breath is mentioned in Koumyou shingon kanju youmon jou.

Aun was such a significant Buddhist concept that it affected most of Japanese culture. During the second World War the Japanese language textbooks for elementary school began with Lesson 1 which read: "Koma Inu-san, a, Koma Inu-san, un."