Microtubular, hairlike structures that some single-celled organisms use for locomotion through liquids. They also cover the cells of certain tissues, such as the epithelium lining the lungs, and help those cells sweep away fluids or particles. Cilia are smaller than flagella.
From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.