The words heroin, heroine sound the same but have different meanings and spellings. Why do heroin, heroine sound the same even though they are completely different words?
The answer is simple: heroin, heroine are homophones of the English language.
A white, odorless, bitter crystalline compound, C17H17NO(C2H3O2)2, that is derived from morphine and is a highly addictive narcotic. Also called diacetylmorphine.
A woman noted for courage and daring action.
A woman noted for special achievement in a particular field.
The principal female character in a novel, poem, or dramatic presentation. See Usage Note at hero.
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").