A completed activity, online or offline, that is important to the success of your business. Examples include a completed sign-up for your email newsletter (a Goal conversion) and a purchase (a transaction, sometimes called an Ecommerce conversion).
A conversion can be a macro conversion or a micro conversion. A macro conversion is typically a completed purchase transaction. In contrast, a micro conversion is a completed activity, such as an email signup, that indicates that the user is moving towards a macro conversion.