A flagellum is part of a cell, a sort of thread that helps the cell move around by moving like a whip. Bacteria are among the cells that can have flagella.
In a bacterial cell, a flagellum twists in a circle like a screw, while in a eukaryote (a plant or animal cell), the flagellum moves in more of an S motion. These long, lash-like appendages propel cells forward, but they also help them sense temperature changes and the presence of certain chemicals. In Latin, flagellum means "whip," from a Proto-Indo-European root meaning "to strike."