A Sacoche (French for satchel) is a crossbody bag, typically thin and with lightweight straps. They’ve become more popular in the biking community for their packability and capacity for quick grocery trips. Some also include an additional third “stabilizing” strap, that prevents the bag from sliding to your front when biking. My version of the bag has that stabilizing strap, plus an interior pocket that the bag can fold itself into and snap closed.