The spinning rainbow-colored wheel that arrives when one of your Mac applications is having trouble responding. It usually only shows up for a few seconds, at which point the application will open or allow you to continue doing whatever high-memory-requiring task you were attempting to do. Also, and more commonly referred to as "spinning beachball", "spinning pinwheel", "beach ball of death/doom".
"Sorry my logos are late. Illustrator kept giving me the gay tornado."