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cilium cil·i·um (sĭl'ē-əm)
n. pl. cil·i·a (-ē-ə)
A microscopic hairlike process extending from the surface of a cell or unicellular organism, capable of rhythmical motion, and acting with other such structures to cause the movement of the cell or of the surrounding medium.
A tiny hairlike projection on the surface of some cells and microscopic organisms, especially protozoans. Cilia are capable of whipping motions and are used by some microorganisms, such as paramecia, for movement. Cilia lining the human respiratory tract act to remove foreign matter from air before it reaches the lungs.