A flagellum ( /fləˈdʒɛləm/; plural: flagella) is a lash-like appendage that protrudes from the cell body of certain prokaryotic and eukaryotic cells. The word flagellum is the Latin word for whip. The canonical role of the flagellum is locomotion but it also often has function as a sensory organelle, being sensitive to chemicals and temperatures outside the cell. Flagella are organelles defined by function rather than structure. There are large differences between different types of flagellum; the prokaryotic and eukaryotic flagella differ greatly in protein composition, structure, and mechanism of propulsion, however both are used for swimming.

An example of a flagellate bacterium is the ulcer-causing Helicobacter pylori, which uses multiple flagella to propel itself through the mucus lining to reach the stomach epithelium. An example of a eukaryotic flagellate cell is the mammalian sperm cell, which uses its flagellum to propel itself through the female reproductive tract. Eukaryotic flagella are structurally identical to eukaryotic cilia, although distinctions are sometimes made according to function and/or length.