The words wain, wane sound the same but have different meanings and spellings. Why do wain, wane sound the same even though they are completely different words?
The answer is simple: wain, wane are homophones of the English language.
A large open farm wagon.
To decrease gradually in size, amount, intensity, or degree; decline.
To exhibit a decreasing illuminated area from full moon to new moon.
To approach an end.
The act or process of gradually declining or diminishing.
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.