It's 18% gray. Of the zones that are discernable on exposed film and paper (B&W specifically) Zone V is at the center. This zone reflects 18% of the light that falls on it. As it is situated in the middle of exposure/print zones/values, it is chosen as a reference point upon which to base exposures should you choose to do so. That's why a card that reflects the same amount of light as a Zone V exposure is referred to as an 18% gray card.