The correct term for referring to the opposite side of a page or paper sheet is usually "the back side" or simply "the back." In some contexts, especially in printing and bookbinding, it can also be called "the verso." Here are a few options:

1. The back side, or the back: Common and straightforward.
2. The reverse side: More formal, often used in instructions or technical contexts.
3. The other side: Informal and commonly understood.
4. Verso: Specifically used in the context of books and manuscripts, where the front side is called "recto."

Choose the term that best fits the formality and context of your situation.