Refers to an automated system that sends out a pre-written message or a series of emails in response to a trigger event. This event could be a new sign up to an opt-in list, an incoming email or a new order for a product/service. You can either install an autoresponder as a piece of software on your own web server for a one-time fee (eg: AutoResponse Plus) or sign up with a company that provides a full-fledged autoresponder service with all the bells and whistles for a nominal monthly fee (eg: GetResponse, Aweber, iContact).