The term 'opposite of NADIR' can be answered with the crossword solution 'ZENITH, ' which is a six-letter word. The term 'ZENITH' refers to the highest point in the sky that a celestial body appears to reach, or the highest point of achievement or development. It is the opposite of 'NADIR, ' which is the lowest point in the sky that a celestial body appears to reach, or the lowest point of achievement or development.