A metric that measures the viral growth rate of an application, defined by how many new users each existing user successfully invites.
Why it matters: If K > 1, the product is undergoing exponential viral growth. If K < 1, viral growth will eventually burn out without paid acquisition.
In practice: If 10 users send out 50 invites total, and 15 new users sign up from those invites, the K-Factor is 1.5 (exponential growth).