Escapism, as in to escape something. This fundamental theme of "The Secret Life of Walter Mitty" is evident throughout the book as Walter fantasizes about amazing and different adventures when in reality most of his life is quite boring. He escapes to this different reality to deal with the issues in his own life. Modern day equivalents for many people include alternate online personalities where shy people can be courageous, much like Walter Mitty's escapes and imaginary adventures.