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
Mull is an Anglicization of the Gaelic Maol, a term for a rounded hill, summit, or mountain, bare of trees (it has also been used, in Gaelic, to refer to a forehead, or to a shaved head). As an adjective, the word is used to indicate something which is bare, dull, or bald.