The Stargazer's initial report on the Maxia Zeta system's planets
The habitable zone was the range of distances from a star at which liquid water could exist on the surface of an orbiting planet or moon.
Maxia Zeta III and IV were located in the Maxia Zeta star system's habitable zone, and were marked as candidates for terraforming when the USS Stargazer surveyed the system in 2355. (TNG: "The Battle" display graphic)
One of Minos Korva's twelve planets was an uninhabited class M planet in the primary habitable zone, as reported by the USS Exeter during a survey in the 2340s. (TNG: "Chain Of Command, Part II")