Signed book with signature on a blank loose sheet

[kena1956]

I recently purchased an Anne Rice book with the author signature on separate sheet of paper inserted into the book. Does anyone know if there is a specialized descriptive term for this unattached page when listing the book for sale?

Also, is this a practice done by the publisher or someone else?

[imagine.ink]

The term you're looking for is "laid in." 

This description is from Biblio:

Laid-in

"Laid In" indicates that there is something which is included with, but not attached to the book, such as a sheet of paper.

The paper item can be a letter, picture, press release, map, or postcard which is loose inside the book. In rare book collecting, an autograph from the author can sometimes be laid-in, increasing the value of the book.

[kena1956]

Thank you so much! I knew there was a term but I couldn't remember what it was. Much appreciated.

[lux-libri]

Laid in if it's loose. Tipped in if it's attached. 

Your signed sheet could have been laid in by a prior owner or by the publisher.  It's common for popular authors to sign blank sheets in advance to be bound in by the publisher during production.  I would check your title to see if other sellers mention a tipped-in signature.  If your signed sheet matches the page size of the book, it's possibly an extra sheet that just wasn't bound in. If it's a smaller size, or on noticeably different paper, it could have been inserted by anyone.