The words pleural, plural sound the same but have different meanings and spellings. Why do pleural, plural sound the same even though they are completely different words?
The answer is simple: pleural, plural are homophones of the English language.
Of, relating to, or affecting the pleura, or the sides of the thorax.
Thin and transparent.
Relating to or composed of more than one member, set, or kind: the plural meanings of a text; a plural society.
Grammar Of or being a grammatical form that designates more than one of the things specified.
Grammar The plural number or form.
Grammar A word or term in the plural form.
Definitions from Wiktionary, Creative Commons Attribution/Share-Alike License, 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").