The nail of the thumb, A small picture, used as a compact representation of a larger image, Computers An image of a graphic or document that is reduced in size so that it serves as an icon of a larger image or to allow multiple images to be viewed on a screen simultaneously or to be transmitted more rapidly, adjective_ Of, relating to, or of the size of a thumbnail, Brief; cursory, transitive verb_ To generate a reduced size image of (a graphic or document), To describe concisely