The term 'BIRD THAT CANNOT FLY' can be summarized by the crossword solution EMU, which is a three-letter word. An EMU is a large, flightless bird native to Australia that is the second-largest living bird by height after the ostrich. It is a member of the ratite family of birds, which also includes the ostrich, rhea, and cassowary. Emus are omnivorous and mainly feed on plants, insects, and small animals. They are also known for their fast running speed, which can reach up to 50 km/h.