A 3D or lenticular cover is a special cover printed with an illusion of depth, or the ability to change or move as the image is viewed from different angles.
3D can also refer to a Hologram Sleeve which uses a different process and has a different result
Can also refer to a 3D sleeve printed in two colours that need special 3D glasses to view

Here is the close up of the comic book surface of a lenticular cover
