noun, plural: flagella
In bacteria, the flagella are helical filaments made up of the protein, flagellin, and rotate like screws. In eukaryotes, such as in cells of animals, plants and protists, they are made up of microtubules surrounded by the plasma membrane and enable the cells to move in a whip-like fashion.
Some flagella are not used for movement but in sensation and signal transduction by various cell types, e.g. rod photoreceptor cells of the eye, olfactory receptor neurons of the nose, kinocilium in cochlea of the ear.
See also: fimbria.