A 3-dimensional solid which is shaped in such a way that its projections along three mutually perpendicular axes are three different letters of the alphabet. Hofstadter (1989) has constructed such a solid for the letters G, E, and B.
See also Cork Plug
References
Hofstadter, D. R. Gödel, Escher, Bach: An Eternal Golden Braid.
New York: Vintage Books, cover and pp. xiv, 1, and 273, 1989.