n. A Middle English form of matter.n. Mother: in certain special uses. See alma mater, and phrases below.n. In anatomy, one of two membranes or meninges of the brain, outer and inner, separated by the arachnoid, and distinguished as dura mater, or dura, and pia mater, or pia: so called from some idea that they produce the brain.—n. The large metallic disk which forms the foundation of an astrolabe, and carries at its circumference the graduation upon which the indications of the alidade or index are read.