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
Meaning: A stiff hollow protective spine on a porcupine or hedgehog. Classified under: Nouns denoting animals. Hypernyms (quill is a kind of...): spine (a sharp rigid animal process or appendage; as a porcupine quill or a ridge on a bone or a ray of a fish fin). Holonyms (quill is a part of...):