When a geometry is described by a set of axioms, the notion of a line is usually left undefined (a so-called primitive object). The properties of lines are then determined by the axioms which refer to them.
Definition of mull 1: friable forest humus that forms a layer of mixed organic matter and mineral soil and merges gradually into the mineral soil beneath 2: a finely powdered solid especially in a suspension
medical Definition of mull : to grind or mix thoroughly (as in a mortar) : pulverize the alloy, after removal from the amalgamator, was mulled in the palm of the hand