The whip-like appendage on some bacteria is called a flagellum (plural: flagella).

Flagella are used for locomotion by bacteria, allowing them to move through liquid environments. They are made up of a protein called flagellin.

Here's a quick summary:

* Function: Movement

* Structure: Whip-like appendage

* Composition: Protein (flagellin)

* Term: Flagellum (plural: flagella)