noun, used as a suffix
west of ...; ... and westward
Attaches to a place name or geographical reference to indicate the area to its west or extending westward from it. Often used in formal or written contexts such as maps, weather reports, and historical descriptions.
関東地方以西では大雨に注意してください。
Please be careful of heavy rain in areas west of the Kanto region.
この路線は東京以西の駅には停まりません。
This line does not stop at stations west of Tokyo.
Compound of 以 (い, 'from; by means of') and 西 (せい, 'west'). The 以 prefix is used in directional suffixes to indicate a reference point and the area beyond it in that direction.