I. thumb ‧ nail 1 /ˈθʌmneɪl/ BrE AmE adjective
thumbnail sketch/portrait a short description that gives only the main facts about a person, thing, or event:
a thumbnail sketch of recent political events in America
II. thumbnail 2 BrE AmE noun [countable]
1 . the nail on your thumb
2 . a small picture of a document on a computer screen, showing you what it will look like when you print it:
Click on the thumbnails to view a larger version of each image.