Author Platform
The audience, visibility, and credibility an author brings to a book, independent of the publisher's own marketing.
An author platform is the reach and credibility an author has built before, during, and after publishing a book: an email list, LinkedIn following, media relationships, speaking history, or professional reputation. Publishers and readers both look to it as a signal that the book will find an audience.
Example
A first-time author with ten years of relevant LinkedIn thought leadership has a stronger platform than someone with the same book idea but no existing audience.
Nuance
A platform is not the same as the book itself. Two people can write similar books with very different outcomes depending on the platform behind each one.