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Ectoderm: The outermost layer of embyonic tissue layers. Becomes the skin and outer covering in animals, and can also include the outer part of the nervous system, external organs and structures.

Efferent: A term referring to movement away from the central part of the body. Ususally used in reference to vessels, nerves, etc. An example would be a nerve impulse from the heart, being sent to the brain.

Electrochemical gradient: A term that refers to the distribution of electrical charge between two adjoining regions.

Electron: A component of the outer portion of an atom that has a negative charge and also provides mass. Compare with neutron and proton.

Embryo: In general, the stage of development for a multicellular organism after cleavage and before birth.

Endoderm: The innermost layer of the embryonic tissue layers, where the digestive tract, repiratory system, and various other systems develop.

Endoparasite: A particular type of parasite that lives inside of its host.

Entire: In plants, this means to have a smooth margin (edge) and can apply to flowers or leaves.

Entomophilous: Flowering plants that are pollinated by insects.

Enzyme: A protein that helps speed up reactions between substances.

Epidermis: The outermost cell layers of any multicellular organism. Such layers can include: hair, claws, nails, hooves, and feathers.

Epiphyte: A plant that grows above ground, and on other organisms or surfaces.

Escape velocity: The minimal velocity (or speed) required to escape a gravitational field.

Estrus: A regular cycle of sexual receptivity in mammalian animals that stimulates sexual reproduction. Also known commonly as being "in heat". Alternatively spelled "oestrus."

Eukaryote: Any cell that has a distinct nucleus along with other well-developed cell structures. Can also refer to any organism that contains such cells.