A 3D display is a type of display that shows images with depth. It does so using depth cues based largely on focus and/or stereopsis.
3D displays can be world-fixed, like mounted on a desk, or they can be head fixed, like built into a VR headset.
To get a 3D display to show proper accommodation cues AKA focus, there are multiple ways. The two main ways are holographic displays and light field displays. A holographic display uses interference patterns to construct light as wave-like movements in an electric field that reach the eye. A light field display sends focal stacks of images based on rays to the eye.
Multifocal displays are displays that show multiple focus images at the same time, so that the eye can choose what to focus on.
3D is more than just stereoscopic flat images. For a few hundred years, a thought was that stereoscopic flat images formed all of 3D vision.
History
2000 to 2010
Autostereoscopic displays from the year 2000 to 2010 used a set of stereoscopic views that were displayed using parallax barriers.