Half turn
Usually done when the man's right foot and the lady's left foot are free. The man steps forward outside right (3 of 8 count basic), leading the lady to step back left, then side right across his right leg, and forward left around him as he shifts weight first to his center, then onto his right foot as he then pivots on both feet, turn with his partner, the lady pivoting on her left foot. Media Vuelta is used by itself to change direction or maneuver on the dance floor and as an entrance to many combinations.
(from tejastango.com (http://www.tejastango.com/terminology.html#M))
The term media vuelta is commonly used to describe a body turn north to south for example, if we call north the direction in which the body is facing before the execution of a media vuelta. Either of both dancers can do a media vuelta at the same time or individually. Visually, a media vuelta is a body rotation over open legs, that is, after the legs open in a forward direction for example, the body turns in the opposite direction of the leading leg. To clarify further, visualize the typical forward ocho. The turning on the forward ocho is in the direction of the leading leg. What differentiates the media vuelta is that it is done with both feet on the ground which allows the dancer to continue moving in either direction, north or south after the turn, depending on how the weight is transferred at the end of the turn. In the case of the man, this allows him to correctly mark the next move for the woman.
(from planet-tango.com (http://www.planet-tango.com/tutorial/dance-14.htm))