The semicircular area in the baulk end of a snooker table is called the D.
The D is drawn behind the baulk line, which crosses the table near one end. At the start of a frame, the cue ball is placed inside this area for the opening shot. It is also used whenever a player has ball in hand after certain fouls or when the cue ball is off the table.
The name comes from the shape formed by the straight baulk line and the semicircular arc. The yellow, brown and green spots lie along the baulk line, with the brown at the centre and the yellow and green on either side.
The D is not a target area or a scoring zone. Its main purpose is to define where the cue ball may be placed in specific situations and to establish the distinctive baulk-end layout.