The words birr, bur, burr sound the same but have different meanings and spellings. Why do birr, bur, burr sound the same even though they are completely different words?
The answer is simple: birr, bur, burr are homophones of the English language.
A whirring sound.
Strong forward momentum; driving force.
To make a whirring sound.
See Table at currency.
A rough prickly husk or covering surrounding the seeds or fruits of plants such as the chestnut or the burdock.
A plant producing such husks or coverings.
A persistently clinging or nettlesome person or thing.
A rough protuberance, especially a burl on a tree.
A rough edge or area remaining on material, such as metal, after it has been cast, cut, or drilled.
Variant of bur1.
To form a burr on.
To remove burrs from.
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").