organ -- Collection of tissues which performs a particular function or set of functions in an animal or plant's body. The heart, brain, and skin are three organs found in most animals. The leaf, stem, and root are three organs found in most plants. Organs are composed of tissues, and may be organized into larger organ systems.
organelle -- n. A membrane-bound structure in a eukaryotic cell that partitions the cell into regions which carry out different cellular functions, e.g., mitochondria, endoplasmic reticulum, lysosomes.
osculum -- The main opening through which filtered water is discharged. Found in sponges.
outcrop -- n. Any place where bedrock is visible on the surface of the Earth. v. The exposing of bedrock at the Earth's surface.
outgroup -- In a cladistic analysis, any taxon used to help resolve the polarity of characters, and which is hypothesized to be less closely related to each of the taxa under consideration than any are to each other.
ovary -- In flowering plants, the part of the flower which encloses the ovules. When the ovary matures, it becomes the fruit.
ovulation -- n. The process by which an egg (the female gamete) is released from the ovary. In animals other than mammals, with the exception of monotremes, this results in the laying of an egg outside of the body. When female mammals ovulate, the egg, if fertilized, is retained within the uterus.
ovule -- In seed plants, the structure which gives rise to the seed.