Translation guide
Refers to the Seto Inland Sea (瀬戸内海), the body of water separating Honshu, Shikoku, and Kyushu. In English, 'Inland Sea' almost always refers to this specific Japanese sea, so the direct translation is the proper noun.
Referring to the specific sea in Japan
The standard Japanese name for the Seto Inland Sea. Use this in all contexts when referring to this body of water.
瀬戸内海は穏やかな海で知られています。
The Seto Inland Sea is known for its calm waters.
瀬戸内海には多くの島があります。
There are many islands in the Seto Inland Sea.
Referring to any inland sea, not specifically the Seto Inland Sea
The generic term for an inland sea. Use this when talking about inland seas in general, not the specific Japanese one.
地中海は内海の一種です。
The Mediterranean Sea is a type of inland sea.
A more technical term for inland sea, often used in geography. Less common in everyday speech.
カスピ海は世界最大の内陸海です。
The Caspian Sea is the world's largest inland sea.
In English, 'Inland Sea' capitalized usually means the Seto Inland Sea. In Japanese, 瀬戸内海 is always the proper noun. Use 内海 for the generic term.