What does cilium mean?
Cilium is the singular form of 'cilia' and refers to one of the many microscopic, hair-like projections that extend from the surface of eukaryotic cells. Each cilium contains an organised core of microtubules and is capable of rhythmic, whip-like movement. In multicellular organisms, motile cilia beat in coordinated waves to transport fluids and particles: in the human airway they sweep mucus upward, clearing dust and microbes from the lungs. A modified, non-motile form called the primary cilium acts instead as a cellular antenna, detecting chemical and mechanical signals from the environment. In single-celled organisms such as paramecia, dense carpets of cilia propel the organism through water. The term belongs firmly to scientific and medical writing and is rarely encountered outside textbooks, though its plural is common enough in popular science coverage of health topics such as smoking and cystic fibrosis.
nounA cilium is a microscopic, hair-like projection from the surface of a cell that beats rhythmically to move fluid, mucus, or debris across tissue surfaces, or to propel single-celled organisms through water.
- A short, hair-like organelle projecting from a eukaryotic cell, beating in coordinated waves to move fluid or propel the cell.
- (In anatomy/zoology) Any of the minute hair-like structures collectively covering certain tissues or organisms.
"Each epithelial cell in the airway carries hundreds of cilia that sweep mucus toward the throat."
"The paramecium glided smoothly across the slide, driven by thousands of beating cilia."
"Damage to respiratory cilia from smoking leaves the lungs unable to clear inhaled particles."
Cilium is nearly always discussed in the plural; 'a cilium' is used mainly when referring to an individual structure.
"The cilia lining the trachea beat about ten times per second."
Your airways are lined with millions of tiny beating hairs — without them, every breath would leave you drowning in your own mucus.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of cilium
Cilium comes directly from Latin 'cilium', meaning eyelid or eyelash, which itself derives from 'cilare' ('to move quickly') and relates to 'supercilium' ('eyebrow'). The word entered scientific English in the 19th century as microscopists needed a precise Latin-based label for these lash-like cell structures, echoing the earlier anatomical use of 'ciliary'. Related English terms include 'ciliary' and 'ciliated', both built from the same root.
Related word forms
How cilium is actually used
Cilium is almost exclusively used in scientific and medical registers; general audiences encounter it far more often in its plural form 'cilia'. It is a technical term of cell biology and histology.
Easily confused with cilium
A flagellum is a longer, whip-like cellular appendage typically used for propulsion, while a cilium is shorter and usually occurs in large numbers on a cell surface.
A microvillus is a tiny non-motile finger-like cell projection that increases absorptive surface area, whereas a cilium is motile and actively beats to move substances.