A flagellum (/fləˈɛləm/; plural: flagella) is a lash-like appendage that protrudes from the cell body of certain cells termed as flagellates. A flagellate can have one or several flagella. The primary function of a flagellum is that of locomotion, but it also often functions as a sensory organelle, being sensitive to chemicals and temperatures outside the cell.[1][2][3][4]

Structure of bacterial flagellum.
SEM image of flagellated Chlamydomonas sp. (10000×)
Anatomical terminology

Flagella are organelles defined by function rather than structure. Flagella vary greatly among the three domains of life, bacteria, archaea, and eukaryotes. All three kinds of flagella can be used for swimming but they differ greatly in protein composition, structure, and mechanism of propulsion. The word flagellum in Latin means whip. The flagella of archaea have a special name, archaellum, to emphasize its difference from bacterial flagella.[5][6]

An example of a flagellated bacterium is the ulcer-causing Helicobacter pylori, which uses multiple flagella to propel itself through the mucus lining to reach the stomach epithelium.[7] An example of a eukaryotic flagellate cell is the mammalian sperm cell, which uses its flagellum to propel itself through the female reproductive tract.[8] Eukaryotic flagella are structurally identical to eukaryotic cilia, although distinctions are sometimes made according to function or length.[9] Prokaryotic fimbriae and pili are also thin appendages, but have different functions and are usually smaller.