Cilium (labels) - illustration

A cilium (plural cilia) is an organelle found in eukaryotic cells. Cilia are thin, tail-like projections extending approximately 5-10 micrometers outwards from the cell body. There are two types of cilia: motile cilia, which constantly beat in a single direction, and non-motile cilia, which typically serve as sensory organelles

Illustration courtesy of Wikimedia Commons and user LadyOfHats (Mariana Ruiz).