What does cilia mean?
Cilia are microscopic, hairlike projections that extend from the surface of many cells. In humans, they are best known for their role in the respiratory tract, where millions of them beat in coordinated waves to move mucus, dust, and microbes out of the lungs; elsewhere in the body they serve sensory and signaling functions. Many single-celled organisms, such as paramecia, also use cilia to swim through water. The word comes from the Latin cilium, meaning "eyelid" or "eyelash," and originally referred literally to those short lashes — a sense still echoed when anatomists describe them as eyelid hairs. Because the word is inherently plural (the singular being cilium), it appears almost exclusively in scientific, medical, and educational writing. Understanding cilia is useful not only for grasping basic cell biology but also for making sense of conditions like smoking-related lung damage, where these delicate structures are destroyed and the airways lose their natural cleaning mechanism.
nounCilia are the tiny, hairlike projections that line the surfaces of certain cells in the body — most famously in the respiratory tract, where they sweep mucus and debris away from the lungs — as well as the short hairs of the eyelids; in many organisms they also serve to propel movement.
- Tiny hairlike organelles projecting from the surface of cells, used for movement of fluid or of the organism itself (biology)
- The eyelashes or similar short hairs (anatomy, chiefly historical/literal sense)
"The cilia lining her airways swept the inhaled dust upward, protecting her lungs."
"Damaged cilia in the airways can allow mucus and bacteria to accumulate, leading to infection."
"Under the microscope, the paramecium's cilia beat in coordinated waves, propelling it through the water."
Because cilia is itself the plural form, it takes plural agreement: "the cilia are," never "the cilia is." It is a plural-only noun borrowed directly from Latin.
"The cilia lining the bronchi were visibly shortened under the microscope."
Every blink you take depends on cilia — and the ones in your lungs beat roughly ten times a second your whole life, without you ever noticing.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of cilia
The word cilia derives directly from the Latin cilium, meaning "eyelid" or "eyelash," and entered English in the early eighteenth century as a term for eyelash-like hairs. When microscopists later observed similar hairlike structures on cells, the name was extended to them. The same Latin root gives us supercilium ("eyebrow") and, through it, the English words "supercilious" and "supercilium," linking cilia etymologically to expressions of haughty raised eyebrows.
How cilia is actually used
Always used in the plural; the singular is "cilium," which is rare outside technical contexts. The word belongs chiefly to scientific and medical register and is rarely used in everyday conversation except when discussing biology or health.
Easily confused with cilia
"Cilium" is the singular form referring to one hairlike projection, while "cilia" is its plural.
"Ciliary" is an adjective describing things relating to cilia (as in the ciliary body of the eye), whereas "cilia" is the plural noun naming the structures themselves.