The words borough, burro, burrow sound the same but have different meanings and spellings. Why do borough, burro, burrow sound the same even though they are completely different words?
The answer is simple: borough, burro, burrow are homophones of the English language.
A self-governing incorporated town in some U.S. states, such as New Jersey.
One of the five administrative units of New York City.
A civil division of the state of Alaska that is the equivalent of a county in most other U.S. states.
Chiefly British A town having a municipal corporation and certain rights, such as self-government.
A small donkey, especially one used as a pack animal.
A hole or tunnel dug in the ground by a small animal, such as a rabbit or mole, for habitation or refuge.
A narrow or snug place.
To dig a hole or tunnel for habitation or refuge.
To live or hide in such a place.
Definitions from The American Heritage® Dictionary of the English Language, 4th Edition and Wordnik.
Homophones (literally "same sound") are usually defined as words that share the same pronunciation, regardless of how they are spelled.
If they are spelled the same then they are also homographs (and homonyms); if they are spelled differently then they are also heterographs (literally "different writing").