The words lakh, loch, lock, lough sound the same but have different meanings and spellings. Why do lakh, loch, lock, lough sound the same even though they are completely different words?
The answer is simple: lakh, loch, lock, lough are homophones of the English language.
One hundred thousand, used especially of units of money.
Scots A lake.
Scots An arm of the sea similar to a fjord.
A device operated by a key, combination, or keycard and used, as on a door, for holding, closing, or securing.
A section of a waterway, such as a canal, closed off with gates, in which vessels in transit are raised or lowered by raising or lowering the water level of that section.
A mechanism in a firearm for exploding the charge.
An interlocking or entanglement of elements or parts.
Irish A lake.
Irish A bay or an inlet of the sea.
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").