thumb·nail / ˈ[unvoicedth]əmˌnāl/ • n. 1. the nail of the thumb.2. [usu. as adj.] a very small or concise description, representation, or summary: a thumbnail sketch. ∎  Comput. a small picture of an image or page layout.

thumbnail A small (lower resolution) reproduction of an image or page, used as a link to the image or page in a Web site or electronic document. Thumbnails are used particularly for cataloguing photographs.