A large body of water on the Earth's surface. Unlike the bodies of water on the inside of a landmass (lakes, ponds, etc.) the water in the ocean has higher salinity. There's usually a beach near the coastline of a landmass.
Despite covering over 70% of the planet, the oceans are one of the least explored areas on Earth, approximately around 95% remains uncharted.
Sometimes one can see the horizon if they look far enough.