The words lac, lack sound the same but have different meanings and spellings. Why do lac, lack sound the same even though they are completely different words?
The answer is simple: lac, lack are homophones of the English language.
A resinous secretion of the lac insect deposited on trees and used in making shellac.
Deficiency or absence: Lack of funding brought the project to a halt.
A particular deficiency or absence: Owing to a lack of supporters, the reforms did not succeed.
To be without or in need of: lacked the strength to lift the box.
To be missing or deficient: We suspected that he was lying, but proof was lacking.
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").