On Tumblr, the term "invisible followers" refers to followers who do not show up on the list of followers publicly visible on a Tumblr blog. In other words, these followers are not visible to the blog owner or other users when viewing the follower count or list on the blog's public page.
The concept of invisible followers emerged due to certain privacy settings on Tumblr. Users have the option to enable a setting called "Hide your blog from search results" in their blog settings. When this setting is enabled, the blog becomes unsearchable and does not appear in Tumblr's search results or directory. Consequently, the followers of such a blog are considered invisible because their presence is not publicly revealed on the blog.