The largest toe (usually) on each human foot (i.e. the toe closest to the other foot when the feet are flat on the ground, side by side, and the legs not crossed) or on a foot of certain other animals.
The fingernail on the thumb.
A rough sketch (e.g., the size of one's thumbnail).
A small picture, used as a compact representation of a larger image.
To describe concisely.
To create a smaller representation of (a larger image).