Medium (or traffic medium) is the traffic source category as defined by Google. It also includes traffic medium tracked via the utm_medium parameter. For example, In the case of google / organic, 'organic' is the medium. In the case of bing / cpc, 'cpc' is the medium. In the case of tripadvisor / referral, 'referral' is the medium. In Google Analytics, medium names are case sensitive. So' email', 'Email' and 'EMAIL' are all treated as different mediums in Google Analytics.