thumbnail

Short definition from Wiktionary

Noun

1. 1
   The fingernail on the thumb.
2. 2
   A rough sketch (e.g., the size of one's thumbnail).
3. 3
   A small picture, used as a compact representation of a larger image.
4. 4
   A picture displayed by posts and videos pre-clicked conveying the matters discussed/shown in that post.

Verb

1. 1
   To describe concisely.
2. 2
   To create a smaller representation of (a larger image).