A whiplike motility appendage present on the surface cells. Prokaryote flagella are composed of a protein called FLAGELLIN. Bacteria can have a single flagellum, a tuft at one pole, or multiple flagella covering the entire surface. In eukaryotes, flagella are threadlike protoplasmic extensions used to propel flagellates and sperm. Flagella have the same basic structure as CILIA but are longer in proportion to the cell bearing them and present in much smaller numbers. (From King & Stansfield, A Dictionary of Genetics, 4th ed)

(flagellum) Whiplike motility appendage present on cell surfaces; eukaryote flagella arise from basal bodies, and are composed of nine pairs of microtubules arrayed around a central pair; flagella are common to all mastigophoran protozoa and occur in such specialized cells as spermatozoa; bacterial flagella are thinner and simpler, composed of strands of flagellin; flagella have the same basic structure as cilia but are longer in proportion to the cell bearing them and present in much smaller numbers.