The words slay, sleigh sound the same but have different meanings and spellings. Why do slay, sleigh sound the same even though they are completely different words?
The answer is simple: slay, sleigh are homophones of the English language.
To kill violently.
Slang To overwhelm, as with laughter or love: Those old jokes still slay me.
A light vehicle mounted on runners and having one or more seats, usually drawn by a horse over snow or ice.
To ride in or drive a sleigh.
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").