The Batsignal is a type of distress signal device. It is a specially modified Klieg searchlight with a stylized symbol of a bat attached to the light so that it projects a large Bat emblem on the sky or buildings of Gotham City. The signal is used by the Police Department as a method of contacting and summoning Batman to their assistance in the event of a serious crisis and as a weapon of psychological intimidation to the numerous villains of Gotham City.
The Batsignal was first used by Commissioner Gordon in order to notify Batman of a puzzle delivered by The Interrogator. After seeing it, Batman expressed his concern that the Commissioner would get in trouble with Mayor Hamilton Hill for building and using it. Barbara Gordon also used it to contact Batman when she realised that an impostor had replaced her father. At this meeting, the signal was partially wrecked when Batman was attacked by a duplicate of Harvey Bullock, and Barbara used Batman's grapple gun to pull the robot into the signal, electrocuting it.
On one occasion, Harvey Bullock was once handcuffed to a pipe on the roof of police headquarters by Lock-Up, and the only way he could call for help was to reach the switch on the signal. Likewise, Bullock used the signal for the first time when reluctantly asking for Batman's help in discovering who was trying to kill him.
In Batman: Mask of the Phantasm, Batman is being hunted by the police on suspicions that he has murdered several gang lords, and Bullock, under orders from Councilman Arthur Reeves, tries to use the Batsignal to lure him into a trap. Batman, knowing that it is in fact a trap, did not respond.