The words ton, tonne, tun sound the same but have different meanings and spellings. Why do ton, tonne, tun sound the same even though they are completely different words?
The answer is simple: ton, tonne, tun are homophones of the English language.
A unit of weight equal to 2,000 pounds (0.907 metric ton or 907.18 kilograms). Also called net ton, short ton.
A unit of weight equal to 2,240 pounds (1.016 metric tons or 1,016.05 kilograms). Also called long ton.
A metric ton. See Table at measurement.
A unit of capacity for cargo in maritime shipping, normally estimated at 40 cubic feet.
A metric ton.
A large cask for liquids, especially wine.
A measure of liquid capacity, especially one equivalent to approximately 252 gallons (954 liters).
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").