The term 'BIG SMILE' can be represented by the crossword solution GRIN, which is a four-letter word. A GRIN is a facial expression that is similar to a smile, but with the corners of the mouth turned up slightly more. It is often used to express amusement or joy. A big smile is a larger version of a GRIN, with the corners of the mouth turned up even more. It is usually used to express happiness or excitement.