The words calculous, calculus sound the same but have different meanings and spellings. Why do calculous, calculus sound the same even though they are completely different words?
The answer is simple: calculous, calculus are homophones of the English language.
Relating to, caused by, or having a calculus or calculi.
Pathology An abnormal concretion in the body, usually formed of mineral salts and found in the gallbladder, kidney, or urinary bladder, for example.
Dentistry See tartar.
Mathematics The branch of mathematics that deals with limits and the differentiation and integration of functions of one or more variables.
Mathematics A method of analysis or calculation using a special symbolic notation.
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.