The tropical zone or tropics was a region on planets and moons near their equators, known for its consistently warm temperatures.
In 2063, Zefram Cochrane claimed that he built his ship, the Phoenix, so that he could retire to a tropical island full of naked women. (Star Trek: First Contact)
In 2153, while visiting the Loque'eque homeworld, Hoshi Sato asked Malcolm Reed if he was okay, to which the overheated officer replied that he didn't care for the tropics. (ENT: "Extinction")
On the heavily glaciated planet Sigma Draconis VI, this zone was ice-free. (TOS: "Spock's Brain")
On the planet Risa, this zone contained jungles. While working for the Ministry of Security in 2135, T'Pol tracked Menos and Jossen, two rogue Vulcan operatives, to the Tropical Zone, where she killed Jossen. (ENT: "The Seventh")