What do the Tropics of Cancer and Capricorn define?

Answer 1

The tropics.

Answer 2

The Tropics of Cancer and Capricorn are imaginary lines located at 23.5 degrees north and south of the equator, respectively. They mark the northernmost and southernmost points where the sun appears directly overhead at least once a year, leading to the longest days (summer solstice) in the Northern Hemisphere and the shortest days (winter solstice) in the Southern Hemisphere.