Sentence:  A space curve contained between two concentric spheres and with its concave side facing always toward the common center of the two spheres is said to be "periplegmatic". The radius vector of a periplegmatic curve is of length between the radii of the two spheres. At the maxima and minima of the radius vector the curve touches respectively the larger or smaller of the two limiting spheres. The radial distance between such a maximum and minimum is called the <WSD>"diastem</WSD>". 
Meaning of the word marked with <WSD> tags:  Intervening space; interval.