Basketball, game played generally indoors by two opposing teams of five players each. Basketball was conceived in 1891 by Dr. James Naismith, a physical education instructor at the YMCA college in Springfield, Mass., as a way to condition outdoor athletes during the winter months. His original list of 13 rules has undergone a century of revision, leading to faster pacing and greater athleticism. Today basketball is one of the most popular American sports and one the rest of the world has adopted.