The term geodesic is a mathematical one. Originally a ‘geodesic’ line was the shortest route between two points on earth. Nowadays, it’s used to describe a tent where the poles criss-cross over the surface, intersecting to form triangles. This distributes the stress across the structure, making it the most stable type of tent for extreme weather conditions. If you climb Everest, chances are that you will want to take a geodesic tent with you.
Semi-geodesic tents use similar principles but generally fewer poles for slightly less extreme conditions. Nevertheless, they are still normally produced in small sizes for those who are likely to pitch them on mountains or in windy, exposed terrain.