Medium is a Dimension in Google Analytics under the Traffic Sources section
Medium Definition: The type of referrals. For manual campaign tracking, it is the value of the utm_medium campaign tracking parameter. For AdWords autotagging, it is cpc. If users came from a search engine detected by Google Analytics, it is organic. If the referrer is not a search engine, it is referral. If users came directly to the property and document.referrer is empty, its value is (none).