The horizon seen where the sea (or large body of water) meets the sky.
船から見ると水平線が遠くに見えた。
From the ship you could see the horizon far away.
夕日が水平線に沈むのを見て、息を飲んだ。
Watching the sunset sink below the horizon took my breath away.
朝の海は穏やかで、水平線がはっきりしていた。
The morning sea was calm and the horizon was clear.