noun, used as a suffix
east of; and eastward
Attaches to a place name or region to mean 'east of that place' or 'the area to the east'. Used in geographical descriptions, weather forecasts, and formal writing.
関東以東の地域では大雪が予想されています。
Heavy snow is expected in areas east of the Kanto region.
この列車は東京以東には行きません。
This train does not go east of Tokyo.
Compound of 以 (i, 'from' or 'by means of') and 東 (tō, 'east'). The 以 prefix is used in similar directional suffixes like 以西, 以南, and 以北.