The words gneiss, nice sound the same but have different meanings and spellings. Why do gneiss, nice sound the same even though they are completely different words?
The answer is simple: gneiss, nice are homophones of the English language.
A banded or foliated metamorphic rock, usually of the same composition as granite.
Pleasing and agreeable in nature: had a nice time.
Having a pleasant or attractive appearance: a nice dress; a nice face.
Exhibiting courtesy and politeness: a nice gesture.
Of good character and reputation; respectable.
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").