If you're looking for an indentation or opening in something, you're looking for a cleft (noun). A person with a cleft (adjective) chin has a little dent in the middle of their chin. Most superheroes have one.
Superman is known for his ability to leap tall buildings, his crush on Lois Lane, and the cleft in his chin. For some reason, that cleft is supposed to make him look strong. Some clefts are not so nice. A cleft palate is an upper lip with a deep indentation in it that should be fixed by a surgeon. You may know the verb cleave, which means to cut down the middle. If you cleave something but don't finish the job, you've probably left a cleft in it.