thumbnail /ˈθʌm.neɪl/noun 4verb 2forms 3 noun4 •The fingernail on the thumb. •A small picture, used as a compact representation of a larger image. •A small picture representing a post or a video in a list. •A rough sketch (e.g., the size of one's thumbnail). verb2 •To create a smaller representation of (a larger image). •To describe concisely. formsthumbnailed · thumbnailing · thumbnails