A firth is a narrow inlet of the sea, sometimes similar to what we call a fjord. It is used mainly in Scotland and northern England to describe long arms of the ocean that reach inland between hills or mountains.

The Firth of Forth in Scotland, for instance, stretches inland from the North Sea near Edinburgh, creating a natural harbor that's been important for shipping and trade for centuries. The Moray Firth, also in Scotland, is home to dolphins that swim in its cold waters.

Some firths form where rivers meet the sea, and they can be wider and deeper than typical river mouths. Many major Scottish cities grew up along firths because these waterways provided safe anchorage for ships and easy access to both ocean trade routes and inland areas. If you visit Scotland, you'll see the word firth on maps frequently, marking where the rugged coastline creates these sheltered waterways between the land and the open sea.