A subcontinent was a large area within a continent.
By the late 24th century, efforts were under way to create a new subcontinent, Atlantis, in the middle of the Atlantic Ocean on Earth. While discussing the project with his brother, Robert Picard stated that he believed Earth didn't need another subcontinent. (TNG: "Family")
A class M planet in orbit of a G-type star was chosen as the new home for the recently awakened Vaadwaur. The planet featured a subcontinent with edible vegetation and an underground water supply. Unfortunately, the region was also subject to windstorms. (VOY: "Dragon's Teeth")
When in orbit of Norcadia, The Doctor chose to spend one afternoon by visiting the Norcadian Museum of Entomology and seeing a "fascinating exhibit of beetle larvae from the equatorial subcontinent." (VOY: "Tsunkatse")
A Dinaali woman who was chief engineer of a water irrigation facility which provided water to half a subcontinent on Dinaal was treated in level blue of Hospital Ship 4-2 in 2377. (VOY: "Critical Care")