The words dine, dyne sound the same but have different meanings and spellings. Why do dine, dyne sound the same even though they are completely different words?
The answer is simple: dine, dyne are homophones of the English language.
To have dinner.
To give dinner to; entertain at dinner: wined and dined the visiting senators.
A centimeter-gram-second unit of force, equal to the force required to impart an acceleration of one centimeter per second per second to a mass of one gram.
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.