Gray cards are used, together with reflective light meters, as a way to produce consistent images in film and photography.
A gray card is a flat object of a neutral gray color that derives from a flat reflectance spectrum. Typical examples are the Kodak R-27 set, which contains 2 8x10" cards and one 4x5" card which have 18% reflectance across the visible spectrum, and a white reverse side which has 90% reflectance, and QPcard 101 and 102 with not only gray surfaces but also white and dark gray. Note that flat spectral reflectance is a stronger condition than appearing neutral; this flatness ensures that the card appears neutral under any illuminant.
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