The words millenary, millinery sound the same but have different meanings and spellings. Why do millenary, millinery sound the same even though they are completely different words?
The answer is simple: millenary, millinery are homophones of the English language.
Of or relating to a thousand, especially to a thousand years; millenarian.
Of or relating to the doctrine of the millennium; millenarian.
Of or relating to millenarians.
A sum or total of one thousand, especially a thousand years.
Articles, especially women's hats, sold by a milliner.
The profession or business of a milliner.
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").