flagellum

1. Biology. A long, threadlike appendage, especially a whiplike extension of certain cells or unicellular organisms that functions as an organ of locomotion.
2. A whip.

[Latin, diminutive of flagrum, whip.]

Hairlike projections that extend from some unicellular organisms and aid in their movement.

(click to enlarge)
The bacterium Proteus vulgaris (greatly magnified) showing flagella (credit: © Lee D. Simon — Photo Researchers)

Hairlike structure that acts mainly as an organelle of movement in the cells of many living organisms. Characteristic of the protozoan group Mastigophora, flagella also occur on the sex cells of algae, fungi (see fungus), mosses, and slime molds. Flagellar motion causes water currents necessary for respiration and circulation in sponges and cnidarians. Most motile bacteria move by means of flagella. The structures and patterns of movement of flagella in prokaryotes differ from those in eukaryotes. See also cilium.

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Pl. flagella [L.] a long, mobile, whiplike appendage arising from a basal body at the surface of a cell, serving as a locomotor organelle; the only known example in biology of a rotatory motion. In eukaryotic cells, flagella contain nine pairs of microtubules arrayed around a central pair; in bacteria, they contain tightly wound strands of flagellin.