Terrestia is the land in which the Geneforge games take place. It is one single continent that spans the entire width of Shaper Territory. It also has several islands sprouting off of it, most notably the Ashen Isles, where Geneforge 3 takes place, and Sucia Island, where Geneforge 1 takes place.
In both G2 and G3 there are references to Terrestia being the smaller of two continents controlled by the Shapers. By G4 references to the second continent disappear, potentially being "retconed" to being just the larger half of Terrestia that is the main stage for G5.