The term 'BIG DIAMOND' can be solved by the three-letter word 'ACE'. An ACE is a playing card with a single pip, or symbol, in the middle. It is the highest-ranking card in most card games and is often used as a wild card. In some games, it can also be used to represent any card of a particular suit. A diamond is a precious gemstone that is cut into many shapes and sizes. A big diamond is one that is larger than average.