The Holographic projector was a type of device that was capable of producing realistic Holograms.
Typically, such devices were mobile allowing them to be easily moved across the battlefield. They were used in various roles from target practice to decoys to distract the enemy.
The Transformers cartoon
Shockwave made use of a holographic projector to test his Sentinel forces on Cybertron. After Bruticus and the Combaticons took control of the Transformer homeworld, Shockwave and Starscream used the device to create holographic invaders that distracted the Combaticons until Swindle uncovered the device.