_Ohm._ An electrical unit of resistance. The resistance of a circuit in which one volt produces one ampere of current. _Oligocene._ Of the Tertiary (geological) period, between the Eocene and Miocene. _Ontogeny._ Individual development. _Opsonic._ Causing bacteria to be consumed more readily by phagocytes. _Optics._ The science of light and vision. _Optimum._ The environment most favorable to the growth of a plant or animal. _Ordovician._ Relating to the Paleozoic (geological) period between the Silurian and the Cambrian. _Ore._ A native mineral from which a metal or metals may be extracted. _Organic._ Living, or derived from a living organism. Relating to all the carbon compounds except a few simple ones. Relating to an organ. Affecting the structure of an organ--of a disease, opposed to _functional_. _Organism._ An organized being, plant or animal. A living individual. _Osmium._ A metallic element. Symbol: Os. _Ovary._ The part of the pistil which contains rudimentary seeds. One of the two reproductive organs in which eggs are produced. _Overtone._ A tone produced by secondary vibrations, as of segments of a piano wire. _Ovum._ An egg or female germ. Plural: ova. _Oxidize._ Combine with oxygen. _Oxygen._ An important gaseous element. Symbol: O. About one-fifth of the air is oxygen. _Paleo-._ In many combinations, old, ancient. Also: palæo-, palaeo-. _Paleozoic._ Of the oldest geological periods. More usually, of the era between the Mesozoic and the Pre-cambrian. The words designating geological eras have been used differently by various scientists. _Paleontology._ The science dealing with life in former geological periods, with species now extinct but made manifest by fossil remains. _Palladium._ A metallic element of the platinum group. Symbol: Pd. _Parallax._ The apparent displacement of an object caused by the actual change in the position of the observer, or by the assumption of two different points of observation. The parallax is expressed in degrees of an angle. _Paralogy._ Mental illness. _Parasite._ A plant or animal that derives nourishment or shelter from another upon which, with which, or within which it lives. _Parthenogenesis._ Reproduction without the union of male and female. _Pascal’s Law._ The law formulated by Pascal which is fundamental for the mechanics of fluids: A fluid under pressure exerts equal force upon equal areas of surface. _Pelagian_ or _pelagic_. Relating to the open sea. Pelagic organisms are those which do not approach the shore. _Penology._ The study of punishment for crime. If this science were studied scientifically, perhaps it would cease to exist. _Perennial._ Lasting for several years; specifically, lasting for more than two years. _Phagocyte._ A leucocyte which consumes disease-producing organisms.