Kefi (pronounced "keh-fee") is a hard-to-translate word. There is no direct English equivalent.
Kefi (also commonly spelled kephi) has been described by various Greeks as meaning the spirit of joy, passion, enthusiasm, high spirits, overpowering emotion or frenzy. Kefi takes many forms and is usually, but not always, associated with the expression of positive emotion.
The custom of smashing plates is considered an expression of kefi when the soul and body are so overwhelmed with an exuberance that you must find an outlet.
So is dancing with a glass of water (or a drink more potent) balanced on the head. The frenzied maenads — in ordinary life, Greek matrons —following Dionysus in ancient times were probably expressing kefi, although a bit bloodier of an expression, as they sometimes tore apart animals they might find on the forested paths. Another example of kefi is the iconic image of Zorba dancing on the beach in Crete in the film "Zorba the Greek," although that, too, carries an undertone of sorrow.
Even so, some Greeks say that kefi is not just something you experience in times of happiness, but it is an energy you maintain even when things are tough. It's dancing in the rain, so to speak. It's a culturally embedded idea to stay positive, euphoric even.
Whether they realize it or not, many tourists to Greece are looking for their own spirit of kefi, which may be found on a kefi-friendly beach or in a Greek taverna.
And those travelers who become "converts" to Greece have almost definitely been infected with the "spirit of Greece," the nearly indefinable concept of kefi.