a dividing membrane or thin partition, commonly with an opening through it.-the muscular and tendinous partition separating the cavity of the chest from that of the abdomen; the midriff.-a calcareous plate which divides the cavity of certain shells into two parts.-a plate with an opening, which is generally circular, used in instruments to cut off marginal portions of a beam of light, as at the focus of a telescope.-a partition in any compartment, for various purposes.