Photo by Chris 73, via Wikimedia Commons
a freewheel is one or more sprockets on the rear wheel that allow coasting. Some people will call this “cassette” which more specifically means a series of sprockets that can be removed as a unit from a ratcheting unit built into the hub. That kind of hub is called a “freehub.”
Others may call the freewheel a “cog” which more generally means a single rear wheel sprocket. A true freewheel has the racheting mechanism built into the sprocket assembly, not the hub, and fastens to the hub with large diameter, closely-spaced threads.