The words lance, launce sound the same but have different meanings and spellings. Why do lance, launce sound the same even though they are completely different words?
The answer is simple: lance, launce are homophones of the English language.
A thrusting weapon with a long wooden shaft and a sharp metal head.
A similar implement for spearing fish.
A cavalry lancer.
Medicine See lancet.
See sand lance.
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.