The words tic, tick sound the same but have different meanings and spellings. Why do tic, tick sound the same even though they are completely different words?
The answer is simple: tic, tick are homophones of the English language.
A habitual spasmodic muscular movement or contraction, usually of the face or extremities.
To have a tic; produce tics.
A light, sharp, clicking sound made repeatedly by a machine, such as a clock.
Chiefly British A moment.
A light mark used to check off or call attention to an item.
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").