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. 1  1: a soft fine sheer fabric of cotton, silk, or rayon. 2  2: an ointment of high melting point intended to be spread on muslin or mull and used like a plaster zinc mull.