The words knight, night sound the same but have different meanings and spellings. Why do knight, night sound the same even though they are completely different words?
The answer is simple: knight, night are homophones of the English language.
A medieval tenant giving military service as a mounted man-at-arms to a feudal landholder.
A medieval gentleman-soldier, usually high-born, raised by a sovereign to privileged military status after training as a page and squire.
A man holding a nonhereditary title conferred by a sovereign in recognition of personal merit or service to the country.
A man belonging to an order or brotherhood.
The period between sunset and sunrise, especially the hours of darkness.
This period considered as a unit of time: for two nights running.
This period considered from its conditions: a rainy night.
The period between dusk and midnight of a given day: either late Thursday night or early Friday morning.
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").