What does Mihaly Csikszentmihalyi mean by Nautilus?
There is lots of opportunity here for jokes about generational differences, as both Csikszentmihalyi and I were born before generational names were even invented, but I will go straight to the answer. It was probably in the 1960s that Nautilus was the brand name of one of the first manufacturers of home exercise/weight machines. In those days there weren't any exercise clubs to join, and, if you didn't like the YMCA scene, then the only option was to buy a set of barbells, which were clumsy and not well adapted to many muscle groups. The Nautilus machine filled that economic gap and became very popular; you see their many descendants in all the exercise clubs today.