The words but, butt sound the same but have different meanings and spellings. Why do but, butt sound the same even though they are completely different words?
The answer is simple: but, butt are homophones of the English language.
On the contrary: the plan caused not prosperity but ruin.
Contrary to expectation; yet: She organized her work but accomplished very little. He is tired but happy.
Usage Problem Used to indicate an exception: No one but she saw the prowler.
With the exception that; except that. Often used with that: would have joined the band but he couldn't spare the time; would have resisted but that they lacked courage.
To hit or push against with the head or horns; ram.
To hit or push something with the head or horns.
To project forward or out.
A push or blow with the head or horns.
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").