Rule 205.3i

Lands have their own unique set of subtypes; these subtypes are called land types. The land types are Cave, Desert, Forest, Gate, Island, Lair, Locus, Mine, Mountain, Plains, Planet, Power-Plant, Sphere, Swamp, Tower, Town, and Urza’s. Of that list, Forest, Island, Mountain, Plains, and Swamp are the basic land types. See rule 305.6.