An absolute reference provides the complete domain address, directory, and filename of a resource, such as an image. For instance, if you had a file named "logo.gif" in a directory called "images" under the domain "http://www.emailonacid.com" you would link to it as follows:

    http://www.emailonacid.com/images/logo.gif

In html context, this might appear as:

    <img src="http://www.emailonacid.com/images/logo.gif" />

Most ESPs require absolute references in your HTML email code.