A conversion is the user carrying out an action you care about: a purchase, a sign-up, a download, completing a form or a phone call. It is the basic unit of success in performance marketing: the reference point for most cost metrics (CPA, CPL, CPO) and for all optimisation. What counts as a conversion is something you define yourself depending on the goal, so it is worth distinguishing main conversions (a sale) from supporting ones (for example a newsletter sign-up). (Compare: Conversion Rate Optimization, CPA)