Apostrophe is a poetic device whereby someone starts talking to an inanimate object that the speaker knows is incapable of understanding, or to a person that the speaker knows isn't present.

From the Greek, 'αποστροφη (or apostrophê for those of you for whom this produces a string of mangled crap). Composed from the roots απω (apô), "away from," and στρεφειν (strephein), "to turn." The idea is that you "turn" "away from" one audience to another one which isn't even there.

Although one might wonder about the mental stability of those who draw too extensively on this device, it's something most of us do anyway, such as when we speak to dead relatives, or to people we know full well are out of range. I have heard it said that, especially in America, people will yell at the players in a football game they are watching on TV. This is a perfect example of apostrophe.