Medium is the type of referrals. When you are using manual campaign tracking methods, the value of utm_medium campaign tracking parameter. When you are using AdWords with auto tagging enabled the value becomes ppc. If the user comes through from organic search the value will be set to organic, it the user came directly to the site the value will be set to (none). This dimension can also be used in the Google Analytics segments feature.

When written in queries it appears as:

ga:medium