A subcontinent was a large part of a continent.
On Earth, India was considered a subcontinent. (citation needed • edit)Robert Picard stated that he believed Earth didn't need another subcontinent, while discussing the New Atlantis Project with his brother. (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")