The phrase "inner diameter" refers to the measurement of the distance across the inside of a circular object or opening. It helps determine the size of the hollow space within something like a pipe or a tube. Full definition
Schmitz found that the outer and inner diameter of this ring, combined with the depth of eye sockets, could closely predict when a bird forages (Vision Research, DOI: 10.1016 / j.visres.2010.03.009). (newscientist.com)
Outer Diameter: 255 mm Inner Diameter: 205 mm Trim: Untrimmed The Reverie Eclipse 255 Carbon Fibre steering wheel is Autoclave cured as a single piece, improving stiffness & reducing inertia. (reverie.ltd.uk)