The words villous, villus sound the same but have different meanings and spellings. Why do villous, villus sound the same even though they are completely different words?
The answer is simple: villous, villus are homophones of the English language.
Biology Of, relating to, resembling, or covered with villi.
Botany Covered with soft, shaggy unmatted hairs.
Biology A minute projection arising from a mucous membrane, especially:
Biology One of the numerous vascular projections of the small intestine.
Biology One of the fingerlike projections of the chorion that contribute to the formation of the placenta in mammals.
Botany A fine, hairlike epidermal outgrowth.
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").