Source / Medium
Source: the origin of your traffic, such as a search engine (for example, google) or a domain (example.com). Medium: the general category of the source, for example, organic search (organic), cost-per-click paid search (cpc), web referral (referral).
Source/Medium is a dimension that combines the dimensions Source and Medium. Examples of Source/Medium include google/organic, example.com/referral, and newsletter9-2014/email.