Light knock is a term used to describe a gentle tapping or rapping sound. It can be used to refer to a light TAP on a door or window, or a soft RAP on a table or other surface. The crossword solutions for the term light knock are 'TAP' and 'RAP', both of which are three letter words. TAP is a verb that means to lightly hit a surface with a finger or hand, while RAP is a verb that means to strike a surface with a quick succession of knocks.