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1. The nail of the thumb.
2. Computers An image of a graphic or document that is reduced in size so that it serves as an icon of a larger image. This allows multiple images to be viewed on a screen simultaneously or to be transmitted more rapidly.
1. Of, relating to, or of the size of a thumbnail.
2. Greatly reduced or limited in scale or scope: a thumbnail sketch; a thumbnail biography.
tr.v. thumb·nailed, thumb·nailing, thumb·nails Computers
To generate a reduced size image of (a graphic or document).