Science: cell biology) 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. Plural of cilium, see cilium. small hair like organs on the surface of some cells, particularly the cells lining the upper respiratory tract. Their wavelike movements waft particles of dust and debris towards the exterior. Some single-celled organisms move by means of cilia. They also move food in the digestive tracts of some invertebrates. Cilia are thin, tail-like projections.