OHP or Overhead Projector is a device that projects images onto a screen for presentations or lectures. However, there are other words that can refer to or describe OHPs. One of these is a transparency projector that projects images from transparencies or slides. Another synonym is a slide projector. This type of projector accommodates 35mm slides that are inserted into the projector and projected onto the screen. For a fixed image or message, a lightbox is another word for an OHP. A lightbox is a flat surface illuminated by a light source that projects the message onto the screen. These are just some of the synonyms for OHP that can be used interchangeably.