This measure is applicable to hexahedral elements only. This is the measurement of the worst ratio of the areas of the opposite faces of the hexahedral element. Ideally, this value should be 1.
In the figure, let A1 = Area of Quad Face {ABDC}, and A2 = Area of Quad Face {EFHG}. If A1>A2, then the Opposite Face Area Ratio of this pair of faces = A1/A2. If A1<A2, then the Opposite Face Area Ratio = A2/A1. Similarly, the Opposite Face Area Ratio is found for each opposing pair of faces, and the maximum of all three pairs is found as the measurement for this hexahedral element.