The term 'The Best of All Possible Worlds' is an expression coined by the German philosopher Gottfried Leibniz in the 17th century. It refers to the belief that God has chosen the best possible world out of all the possible worlds that could exist. The crossword solution for this term is IDEAL, which means perfect or excellent. Therefore, the term 'The Best of All Possible Worlds' can be summarized as a belief that God has chosen the perfect world out of all the possible worlds that could exist.