O

oblanceolate— Inversely lanceolate.

obovate— Reverse of ovate, the terminal half broader. (See Fig. 6.1.)

obsidian— A naturally occurring glass of volcanic origin.

obsidian hydration— Dating method in which the thickness of a microscopic "hydration" layer is used to determine the age of a piece of obsidian.

opportunistic— Taking advantage of what is available.

order of magnitude— Refers to a hierarchy of relative size, with each class about 10 times the size of the next smaller class. Features that have characteristic dimensions such as 10, 100, 1,000 and 10,000 mi are said to be of different orders of magnitude.

orogeny— Process of formation of mountains.

orographic— Pertaining to or caused by mountains.

orthopteran— A member of the Order Orthoptera, insects with biting mouthparts, two pairs of wings or none (e.g., crickets and grasshoppers).

ovary— The part of the pistil that contains the ovules. (See Fig. 6.1.)

ovate— The shape of an outline of a hen's egg, the broader end downward.

ovipositor— A pointed organ located on the anal segment ("tail") of an arthropod, used for laying eggs.

ovoid— Solid ovate or solid oval.

oxidation— A biochemical reaction in which a molecule loses an electron, commonly through addition of an oxygen atom, and during which energy is released.