What is not considered a default “medium” in Google Analytics?

Organic, Referral and None are considered as default medium in Google Analytics.

On the order side, Google is considered a source (and not Medium). According to Google, medium is the general category of the source, such as organic search (organic) and cost-per-click (cpc), referral while the source is the origin of your traffic, like a search engine (for example Google and Bing) or a domain (databloo.com). 

Examples of default Source & Mediums in Google Analytics

Definitions of default Mediums in Google Analytics

1. Organic represents traffic that comes from organic search results.
2. Referral shows traffic that comes to your site from other websites.
3. (none) is the direct traffic to your website from users type your URL or click on a bookmark.