In baseball, a moonshot is referred to as a home run that travels a great distance vertically, while only barely clearing the fences. The name "Moonshot" comes from Wally Moon whose home runs would be referenced in newspapers as "Moon Shots". Most of Moon's home runs came at the L.A. Coliseum and gained (perhaps overstated) recognition for hitting home runs to the opposite field, where the distance of the fence was merely 251 feet from home plate, but towered 42 feet high - for comparison, Fenway Park's famous Green Monster is 37 feet tall.
Another definition comes from the trajectory of the ball as it travels high and deep into the night sky, as if it were metaphorically traveling to the moon.