In baseball, while there are nine named fielding positions, players may move around freely. The positioning for the nine positions is very flexible, although they all have regular depths—distances from home plate, and sometimes lateral positioning. A shift means that a player is playing in a noticeably different location than the norm for his actual position.
A fielder who is playing shallow or in is playing closer to home plate, while a player playing deep is playing farther from home plate than normal.
Regular terms are used for some positionings, for example, double play depth is used when there is a force play at second base. This means the shortstop and second baseman are playing slightly closer to second base and sometimes a little bit shallower. This position makes it easier to turn the double play. Bringing the corners in means the first and third baseman are both playing in, this will often be used with runners at third base.
Sometimes in the bottom half of the ninth inning (or later), when a team has a man on third base and less than two outs, the defending team will pull the outfields in very far, almost creating three extra infielders. This is sometimes known as do or die depth.
There are also some very irregular positionings. For example, versus excellent left-handed pull-hitters like Ted Williams and Barry Bonds, teams will move more players to the right side of the field. They sometimes play with the shortstop behind or even to the right of second base. The second baseman may simply move to the right, in which case this just qualifies as a shift, or he might move deeper into shallow right field—this particular formation is called the wishbone defense, because of the shape the shortstop, first baseman, second baseman, and right fielder make.