The term 'THE LONELIEST NUMERO?' is solved by the crossword solution UNO, which is a three-letter word. UNO is a card game that originated in Italy and is now popular around the world. The game is played with a special deck of cards, and the goal is to be the first player to get rid of all their cards. The card with the lowest value is the 'LONELY' UNO card, which is why it is referred to as the loneliest numero.