Final answer:
The MRC defines video viewability as the measurement of whether a video advertisement is actually seen by a user.
Step-by-step explanation:
The MRC (Media Rating Council) defines video viewability as the measurement of whether a video advertisement is actually seen by a user. According to the MRC, a video ad is considered viewable if at least 50% of the ad's pixels are in the viewable area of the browser window for at least 2 consecutive seconds. Viewability is an important metric for advertisers to ensure that their ads are being seen by their target audience.