Why is vitamin C called an antioxidant?
Ascorbic acid is considered an antioxidant because it can supply an electron to a species with an unpaired electron (a "radical", labeled "X" in the below diagram) to stabilize the radical. Of course, since ascorbic acid has now lost an electron, it is itself a radical - but it is a very stable radical due to resonance. Below I have included a diagram of this process below, from UC Davis.



Note how the we can "electron-push" to get the unpaired electron in the middle structure to move to the oxygen on the other side of the ring. This resonance delocalization of the unpaired electron in the ascorbyl radical makes it much more favorable to have the unpaired electron in the ascorbyl radical rather than the X.

Because radicals normally poach electrons from surrounding molecules to pair with their lone electron, they normally cause a chain reaction where their "victim" goes on to pull electrons from another neighboring molecule (oxidation), and so on and so forth. However, because the ascorbyl radical is stable, it is less likely to poach an electron from another molecule in its environment, preventing the chain reaction from occurring. Thus, it is an antioxidant.