Sprites are huge flashes of electricity in the upper atmosphere many kilometers above thunderstorm activity. They constitute a phenomenon defined only recently and still not well understood. Tens of kilometers across, they can look like titanic, glowing jellyfish or take on a more ball-like appearance. Geophysicist Colin Price suggests they may be behind some otherwise puzzling UFO reports.
COMMENT: Having read a lot of the UFO literature, I can think of some cases this applies to. Some witnesses could have been too far away to see the storm activity below, but able to see the gigantic sprites. With no frame of reference in the upper atmosphere, a sprite could certainly look like a smaller glowing object closer to the observer. Still, Sprites don't last long and don't move, so they certainly don't fit many other reports that are still in the "unexplained" file.
This does remind me yet again of a theorized phenomenon, suggested by the late Philip Klass and others, that I strongly suspect is real and is behind many UFOs: a larger, longer-lived cousin of ball lighting that can appear without the need for nearby storm activity. It's uncertain just what the mechanism would be, but there's no agreement on the mechanism behind regular ball lighting either.