Shikishi 色紙 (しきし) is a particular format of paperboard for art, calligraphy, poetry and autographs. Recognizable by its squarish dimensions, it also has characteristic edges which are often evident in posts that are photos of traditional media pieces.
As it is popularly used by artists drawing sketches to give away at conventions (e.g. pool #1135), the term is sometimes used to describe such drawings as well. These are often autographed, dated and/or inscribed with the name of the convention (and artist's booth number). Marker is a common medium for these.
Sanseido Daijirin gives two dimensions for shikishi: 19.4×17 cm and 18.2×16 cm. Some pixiv taggers will use 「小色紙」 ("small shikishi") for the smaller size. Variants may be colored, have pre-printed elements, or may even have gold or silver leaf embedded in the paper. Fancy novelty types are sometimes used; these may come in a different shape (post #1952713) or expose a differently shaped drawing area instead of the full card (e.g. post #523719).
Not to be confused with irogami 色紙 (いろがみ), colored paper commonly used for crafts and origami.
For some 'shikishi' present in drawn images, use shikishi (object).