Readership is another way to talk about all the readers of a particular book or periodical. If your online magazine has a readership of five, and one of them is your mom, it's not a roaring success.
A newspaper, website, or author's readership is their audience — it's the group of people who regularly read their publication. The local zine writer might have a readership in the dozens, while the Harry Potter books have a vast readership, numbering in the millions. When newspapers and magazines started to lose their readerships, many turned their attention to online versions.