Cilium and Flagellum

Cilium noun - A hairlike projection from the surface of a cell; provides locomotion in free-swimming unicellular organisms.

Flagellum and cilium are semantically related. In some cases you can replace term "Flagellum" with "Cilium", this nouns are similar.

Flagellum noun - A lash-like appendage used for locomotion (e.g., in sperm cells and some bacteria and protozoa).

Cilium and flagellum are semantically related. You can use "Cilium" instead a noun "Flagellum".