A suffix meaning 'along' (used after a noun for things like a river, coast, or railway); usually appears as ~に沿い or ~沿い.
川沿いの道を散歩した。
I walked along the riverside path.
線路沿いに新しい商店ができた。
A new shop opened along the railway.
海沿いのホテルから朝日がきれいに見える。
You can see a beautiful sunrise from the hotel by the sea.