"I THINK" a glider is a plane that once released from its method of launch, the flight will be a decending one with little ability to use lift from thermals. As a kid I flew full sized "gliders." They were open cockpit, tandem, and were nicknamed flying bricks. We were winch launched to about 800ft, and once off the winch it was a really quick flight, but perfect for training. Later I went to sailplanes. Sailplanes are able to use thermals to maintain or gain altitude. I could also have dreamed this up (except me flying).