The time-honored and widely used expression "flat film (or plate) of the abdomen" is an unpopular one among radiologists generally. It deserves to be much more unpopular than it is, however, because there is no general agreement as to what it means.
Its origin is lost in the comparative antiquity of the past half-century. German radiology has no comparable term, and it seems likely that the expression originated in America. Holmes1 says he recalls its gradually increasing popularity at the Massachusetts General Hospital a few decades ago. The word "plate" referred, of course, to the glass plates used for roentgenograms prior to the introduction of flexible film in 1918. The question is, what was—and is—meant by "flat"?
Dorland's Medical Dictionary defines "flat" somewhat inadequately as meaning "1. Lying in one plane; having an even surface. 2. Having little or no resonance. 3. Slightly below the normal pitch of sound."
Arnold HL, Washko PJ. "FLAT FILM" OF THE ABDOMEN. JAMA. 1958;167(6):773-774. doi:10.1001/jama.1958.72990230027021