A frenulum (or frenum) is a small fold of tissue that prevents an organ in the body from moving too far.
Cover crops usually are grown to prevent soil loss from wind and water erosion. Use fast-growing cover crops, such as winter wheat or annual rye, on fall-spaded gardens. A second, and probably more important reason home gardeners should use cover crops is to improve soil structure and increase organic matter.
A fallow crop is a crop that you plant usually in the summer/fall and let it there to act as a green manure for the soil. You can harvest top growth for forage or feed but the vines and roots help build the soil tilth. If you leave the entire plant would then technically be called a cover crop, I think.