A basketball phrase which refers to the area of the court where a defender is in a vulnerable position compared to his offensive opponent. A defender always wants to make sure he is between his man and the basket, giving enough space for his opponent to move, but without giving up valuable position closer to the hoop. If too much space is given between the defender and his opponent, or between the defender and the hoop, there is greater potential for the defender to be caught out of position by the offense.
Another phrase which refers to the same area on the court is "no-man's land".
This term comes from the desert found in the United States, Death Valley-an inhospitable desert known for being extremely hot and dry. On the basketball court, it is important for a defender to stay in position. If a single defender is caught out of position, the likelihood of a score by the offense increases exponentially. That is why coaches refer to being out of position as being in Death Valley-it is an extremely bad place to be.