In baseball, “on-deck hitter” refers to the player who is batting next (after the batter who is at-bat).
In a professional baseball game, the on-deck batter traditionally waits inside the 3-ft diameter “on-deck circle” which can be a simple outline or a piece of turf.
There are two on-deck circles in all MLB stadiums.
Some on-deck circles are awfully close to the batter’s box which means there is a good chance of getting hit by an errant foul ball so MLB players on-deck pay close attention.
During the Miami Marlin @ Texas Rangers game, Adrian Beltre actually got tossed in the 8th inning after Beltre moved the on-deck circle further away from the batter box (Beltre was warming up further off the on-deck circle when the umpire told Beltre to warm up inside the circle. Beltre felt that it was too close to the batter’s box so he simply picked up the on-deck circle matt and moved it further away, resulting him being tossed).
Due to safety concerns, most youth fields have on-deck “circles” behind a protective fence to protect a player from an errant ball.
If particular field does not have this separate area, check with the league to see if there is a designated warm-up area. You will probably need to assign a coach to monitor in this situation to minimize the risk of injury.
The player next in line behind the on deck batter is referred to as being in-the-hole.
This term is primarily used in youth baseball because the in-the-hole player is responsible for retrieving a bat used by the previous batter (in-the-hole player should wear a helmet when getting on the field).
If you are a bench coach, you know that kids are notorious for not paying attention to the batting order so you want to make sure the game moves along by reminding players with “who’s on deck” and “who is in the hole” questions.