What is an antonym for fjord?

There isn't a perfect antonym for "fjord" in the sense of a direct opposite. However, depending on the context, you could use several words to express the opposite concept:

* Bay: A bay is a broad inlet of the sea, generally smaller and less dramatic than a fjord.

* Gulf: Similar to a bay, but larger and often with a wider opening.

* River valley: A valley carved by a river, generally with gentler slopes and not formed by glaciers.

* Plain: A vast, flat expanse of land, contrasting with the steep, narrow nature of a fjord.

* Mesa: A flat-topped hill with steep sides, the opposite of the elongated, narrow shape of a fjord.

* Open sea: This emphasizes the opposite of the enclosed, sheltered nature of a fjord.

The best word to use depends on the specific situation and the specific aspect of a fjord you want to contrast.