What is a "flight wheel"?

At Planetary.org I read this:

Below is a photo of a flight wheel -- that is, one of the six that's sitting on Mars today.

Never heard flight wheel before and couldn't find anything when I searched for the expression. To me it looks like a regular wheel (how regular a wheel that is used on a vehicle on Mars can be).

Edit: and, BTW, I associated to "fly wheel", which seems far fetched and probably isn't correct.

Solution 1:

A 1996 NASA document (Wheel Abrasion Experiment Metals Selection for Mars Pathfinder Mission) explains the jargon, albeit obliquely:

The final flight hardware was built using three strips of 707 l-T6 aluminum ... A WAE flight wheel is shown in figure 5.

A flight wheel is one fitted for the flight, that is, the final version from development and testing.