The term 'THAT's Impressive!' can be expressed in a crossword puzzle using the solutions WOW (3 letters), OOH (3 letters), GEE (3 letters), and ILLBE (5 letters). These words are all interjections THAT are used to express surprise, admiration, or awe. WOW is an expression of strong admiration, OOH is an expression of surprise, GEE is an expression of amazement, and ILLBE is an expression of admiration. Together, these words express the sentiment of 'THAT's Impressive!'