Terminology: the reciprocal of sphericity (i.e., actual particle surface area divided by surface area of the equal-volume sphere) is commonly called the ________.

Introduction / Context:
Shape descriptors appear in correlations for drag, pressure drop, and heat/mass transfer. Two complementary terms are sphericity (≤1) and its reciprocal (≥1).

Concept / Approach:
Shape factor is often defined as actual particle surface area divided by the surface area of a sphere having the same volume, i.e., the reciprocal of sphericity. This usage distinguishes it from other “specific surface” or “Sauter diameter” terms, which relate size and area but not shape relative to a sphere.

Final Answer:
Shape factor