A cilium is an organelle found in eukaryotic cells. Cilia are thick protuberances that project from the much larger cell body.

There are two types of cilia: motile cilia and nonmotile, or primary cilia, which typically serve as sensory organelles. In eukaryotes, motile cilia and flagella together make up a group of organelles known as undulipodia. Eukaryotic cilia are structurally identical to eukaryotic flagella, although distinctions are sometimes made according to function and/or length. Biologists have various ideas about how the various flagella may have evolved. (source)