a television audience especially with respect to size or makeup.
They’re called Nielsen Ratings . They extrapolate numbers from there, so, for every one Nielsen household that watches a show they extrapolate it to mean 1,000 other households watched it, or some number. For more information visit : How do Networks Know How Many People are Watching a TV Program?
TV ratings are measured by Nielsen, which calculates the live viewership of a particular broadcast based on a metric called “average minute audience.” It’s exactly what it sounds like: the average number of people that watched a TV broadcast during any 60-second portion of that broadcast.