Source is a Dimension in Google Analytics under the Traffic Sources section
Source Definition: The source of referrals. For manual campaign tracking, it is the value of the utm_source campaign tracking parameter. For AdWords autotagging, it is google. If you use neither, it is the domain of the source (e.g., document.referrer) referring the users. It may also contain a port address. If users arrived without a referrer, its value is (direct).