The Overmind Player is the player who represents the Overmind and controls the Alien Forces, trying to invade the Earth and defeat the Hero Players. In the fully co-operative mode, the Overmind Player is replaced by an AI controlled by a deck of cards.
Alien Forces
The Alien Forces are all the Alien Units under the control of the Overmind Player. Sometimes, in story and flavour texts, the rulebook and Mission Book also uses the term a bit more loosely and symbolically.