Spacial Light Modulator: This is a fancy name for a transparency of some kind that light can pass though. Usually when the word SLM is used, it means that the transparency can be changed from a computer. In the 1980s large SLM's were placed on overhead projectors to project computer monitor contents to the screen. Since then more modern projectors have been developed where the SLM is built inside the projector. These are commonly used in meetings of all kinds for presentations.
SLM's are also used extensively in holographic data storage setups to encode information into a laser beam in exactly the same way as a transparency does for an overhead projector.