The words lightening, lightning sound the same but have different meanings and spellings. Why do lightening, lightning sound the same even though they are completely different words?
The answer is simple: lightening, lightning are homophones of the English language.
The removal of cargo (especially crude oil) from a vessel in order to reduce its draft
Present participle of lighten.
The sensation caused by the descent of the uterus into the pelvic cavity before the onset of labour
An abrupt, discontinuous natural electric discharge in the atmosphere.
The visible flash of light accompanying such a discharge.
Informal A sudden, usually improbable stroke of fortune.
To discharge a flash of lightning.
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").