Family. A category used in the classification of organisms that consists of one or several similar or closely related genera. Similar families are grouped into an order. Family names end in aeeae or ae in botany(e.g. Cactaceae ) and idea in zoology (e.g. Equidae). The names are usually derived from a type genus (Cactus and Equns in the example above) that is characteristic of the whole family (see type specimen). In botany, families are sometimes called natural orders.

Fascia. A sheet of fibrous connective tissue occurring beneath the skin and also enveloping glands, vessels, nerves, and forming tendon sheaths.

Fasciation. The growing together of branches or stems to form abnormally thick growths.

Fat.
A mixture of lipids, chiefly triglycerides, that is solid at normal body temperatures. Fats occur widely in plants and animals as a means of storing food energy, having twice the calorific value of carbohydrates. In mammals, fat is deposited in a layer beneath the skin(subcutaneous fat) and deep within the body as a specialized adipose tissue.

Fatigue. Exhaustion in muscles resulting from exertion or over stimulation following a period of activity.

Fatty acid. Organic aliphatic acid. Biological ones have usually unbranched chains and an even number of carbon atoms.

Fauna. The animal population present in a certain place or at a certain epoch.

Favism.A human disease characterized by the destruction of red blood cells, resulting in serve anaemia.

Feather. Any of the flat light waterproof epidermal structures forming the plumage of birds, several types of which form the body covering of birds.

Feather Follicle.
Deep pit in epidermis surrounding base of feather. Wall  of follicle  at bottom of the pit is continuous with base of feather.