The source dimension allows you to obtain the source of the referrals. When you use manual campaign tracking, the value of the utm_source tracking parameter. When you use AdWords with auto tagging enabled the value will return Google. Otherwise you will receive the domain of the source referring the user to your website, the value could potentially also be displayed as a port address, if the user came directly to your site (via no link) the value will be (direct). This dimension can be used in the segmentation feature in Google Analytics.

When written in queries it appears as:

ga:source