- A microscopic hairlike process extending from the surface of a cell or unicellular organism. Capable of rhythmical motion, it acts in unison with other such structures to bring about the movement of the cell or of the surrounding medium.
- An eyelash.
- Botany One of the hairs along the margin or edge of a structure, such as a leaf, usually forming a fringe.
Origin of ciliumLatin eyelid ; see kel-1 in Indo-European roots.