What is the term for the surface along the fillet curves or between the fillet radius of two adjacent gear teeth?

The term that describes the surface along the fillet curves or between the fillet radius of two adjacent gear teeth is known as the "fillet." The fillet is the rounded corner or contour that connects two surfaces. In the context of gear teeth, it serves to reduce stress concentrations and enhance the strength of the tooth profile.

The other terms involve different aspects of gear geometry. For instance, "flank" refers to the side surface of the gear tooth that engages with the mating tooth, while "bottom land" is related to the surface at the bottom of the tooth space, and "top land" pertains to the flat area at the top of the tooth. These terms, while relevant to the overall geometry and function of gears, do not specifically refer to the curved area between adjacent teeth where the fillet is located.