1. The nail of the thumb.
2. Computer Science. A reduced image of a graphic or document page, used in order to view multiple images on a screen simultaneously or to download such images more rapidly.

1. Of, relating to, or of the size of a thumbnail.
2. Brief; cursory: a thumbnail biography.

The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved.