A gulf is a large area of sea partly enclosed by land, such as the Gulf of Mexico, while a bay is a smaller body of water set into a curve of the coastline, such as a sheltered fishing bay. Size and scale are the main clue: gulfs tend to be much bigger.
The core difference
A gulf is a large, often deep area of sea enclosed by land, while a bay is a smaller coastal inlet.
- gulf — a large area of sea partly enclosed by land: the Gulf of Mexico borders several US states.
- bay — a body of water partly enclosed by a curve of the coastline: the fishing boats sheltered in the bay during the storm.
How to tell them apart
Both describe water bordered by land, but a gulf is generally the larger, more enclosed of the two, while a bay is a smaller curve in the coastline, often just big enough to shelter boats. Named examples follow this pattern: the Gulf of Mexico is vast, while a fishing village's bay is modest by comparison.
"Gulf" also has a well-known figurative sense, describing a wide gap or difference between people or groups, as in a growing gulf between rich and poor. "Bay" branches out even further: it can mean a compartment such as a loading bay, the bay tree used for its leaves in cooking, the verb to bay meaning to bark or howl, and even a reddish-brown horse colour.
