Ao (color)
Ao (hiragana あお; kanji 青; adjective form aoi (青い)) is a Japanese color word that includes what English-speakers would call blue and green. For example, in Japan, green traffic lights are described as ao shingō (青信号), and blue skies are described as aozora (青空), as in aozora bunko.
Ao vs. Midori
Modern Japanese has a separate word for green (緑 midori), although its boundaries are not the same as in English. Ancient Japanese did not have this distinction: the word midori only came into use in the Heian period, and at that time (and for a long time thereafter) midori was still considered a shade of ao. Educational materials distinguishing green and blue only came into use after World War II, during the Occupation: thus, even though most Japanese consider them to be green, the word ao is still used to describe certain vegetables, apples and vegetation. Ao is also the name for the color of a traffic light, "green" in English. However, most other objects—a green car, a green sweater, and so forth—will generally be called midori. Japanese people also sometimes use the English word "green" for colors. The language also has several other words meaning specific shades of green and blue.
Other meanings
The color ao can also indicate youth, as in the song "aoi kajitsu" (a song about a young girl) by singer Momoe Yamaguchi, a meaning that stems from the on'yomi (Chinese-based reading) sei of the kanji for ao. Yamaguchi's biography about her youth was called aoi toki (my young days). Fresh fruit in Japan is sometimes written seika (青果).
The word aoi can also mean a hollyhock. In writing, this form of "aoi" appears in Japanese as hiragana あおい, or あふひ in historical kana usage, and kanji 葵.
Ao also appears in the name of Aomori Prefecture.
The main antagonist of Ao Oni is a blue oni, or Japanese ogre of the same name.