What are additional blogs created under one Tumblr account called?

Additional blogs created under one Tumblr account are called secondary blogs.

Every Tumblr account begins with a primary blog, the account’s first blog and the one with the broadest social features. Users can then create extra blogs within the same account; Tumblr officially calls these secondary blogs. Tumblr also uses “sideblog” as an informal alternative term.

Secondary blogs can be customized, followed, reblogged from, and—unlike primary blogs—protected with a password. They can also have multiple members, which turns them into group blogs. However, they cannot perform every action available to a primary blog, such as following other blogs or liking posts.

“Sub-blogs,” “extra blogs,” and “side blogs” may sound reasonable, but only “secondary blogs” is the standard formal label. “Sideblog” is a recognized Tumblr synonym, yet it is not the stored answer and does not replace Tumblr’s canonical terminology.