You might see referral traffic coming from search engines like Google or Bing, a blog, newsletter, or email or social media sites like Facebook. Referrer traffic could be organic or through a paid search like cost-per-click (CPC) advertising or a paid advertising campaign that you have created (but in many cases, the paid advertising will show up in its very own Paid traffic type).
Visitors can also fall into the direct category by clicking a link to your site from an email or PDF document, accessing your site from a shortened URL (which is an abbreviated version of your website address), clicking on a link from a secured site to your non-secure site, or clicking on a link to your site from a social media application like Facebook or Twitter. And, there is a chance that accessing your site from an organic search can end up being reported as direct traffic.
Another point which you did mention (between the lines: "and updating your backlinks to point to HTTPS URLs") but I actually didn't realize enough is that updating backlinks when your site switches from http to https is very important. Redirecting all http traffic to the https version of your site from the server (301) is necessary, but doesn't solve losing referral data. Only updating backlinks, especially those on httpS-sites, does. I'd like to emphasize that!
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Direct traffic is defined as visits with no referring website. When a visitor follows a link from one website to another, the site of origin is considered the referrer. These sites can be search engines, social media, blogs, or other websites that have links to other websites. Direct traffic categorizes visits that do not come from a referring URL.