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1. A small picture that presents a smaller version of a larger image or proposed image. Before fully redesigning the layout of the office, the interior designer provided me with a thumbnail sketch of his initial plans.
2. A short description or preview. Class, each of you will need to submit a thumbnail sketch of your essay topic for my approval.
a thumbnail sketch
a brief or small picture or description. The manager gave a thumbnail sketch of her plans. The student wrote a thumbnail sketch of his project.
A brief outline or cursory description, as in Let me give you a thumbnail sketch of the situation. This idiom alludes to drawing a picture no larger than a thumbnail. [Mid-1800s]
n. a quick and concise description. (One that could be written on someone’s thumbnail.) Let me give you a thumbnail sketch of what happened.