I find that most people just refer to the white light at the back of their boat their 'anchor light'.
On my boat, I have both a stern light, and a masthead light that can double as an anchor light (in addition to the red/green side lights).
Here's an image that shows how the lights can be (I find pictures easier than reading regulations in this case):
So generically, a lot of people just refer to the white light as an anchor light. On the majority of small to average sized boats on the lake, they double as an all around light as well as an anchor light. If they are underway, they need their red/green side or bi-color lights on along with their 'anchor light' (technically it's an all around light). If they are anchored, then the red/green side or bi-color lights must be off.
On my boat, since I have a separate stern light and a 'masthead light', when I switch from 'navigation' to 'anchor' mode. the stern light turns off, and the mast head light changes from mostly forwards facing only to a 360 degree white light.
I don't think people should get too tied up in a bunch when someone says 'your anchor light must be on when underway', as in many cases, the anchor light doubles as an all around light when underway for smaller boats.