A viewmodel is a
model used specifically as the visible portion of the player.
GoldSrc and
Source games generally do not use a full player model for the first-person view, instead only rendering arms and weapons. (For example, when the player is holding a crowbar in Half-Life, the viewmodel is the model of the players hands and the crowbar itself.) Of course, this only applies to oneself; in multiplayer games, the game renders full models for other players.