The completion of a predefined goal, simply it's the action that the campaign is trying to get the user to take. This is often used to track the number of site visitors that have been “converted” into paying customers, though sales are not always chosen as the metric.
Conversions are based on the overall goals of the brand or client. The most common conversion metrics are calls, leads, sign-ups, installs, transactions, newsletter subscriptions, content downloads from the website and reaching a thank you page on a website