www.google.com

What it is

Organic clicks from Google Search, including Google Images, which uses the same referrer. Since 2011 the referrer hasn't included the search query, which is why keyword reports filled with "(not provided)".

Why there's no full URL

Google moved search to HTTPS and stopped passing queries in the referrer. Sites now get only Google's home page as an origin referrer, and Google points site owners to Search Console for query data.

What traffic it represents

Visits from Google's organic results, on any Google country domain.

Sources

for organic search results on SSL search, a web site will only know that the user came from google.com

Accessing search query data for your sites - Google Search Central Blog

websites you visit from our organic search listings will still know that you came from Google, but won't receive information about each individual query.

Making search more secure - Official Google Blog

You may start to see origin referrers—Google's home pages (see the meta referrer specification for further detail)—as a source of organic SSL search traffic.

Upcoming changes in Google's HTTP Referrer - Google Search Central Blog

we will continue to use https://www.google.com (or the appropriate ccTLD) as the referrer URL for all traffic from Google Images

An update to referral source URLs for Google Images - Google Search Central Blog