What is a single mirror?

Textbooks and other technical reference manuals are often full of jargon and technical terms. The authors don't take time to carefully define these terms. Understanding the exact meaning is critical to understanding the material.
In this case, the author is not discussing some special type of mirror. The word "single" means that there is only one mirror.
If this term was used in a question in your textbook, it might have said something like: "How can you change the direction of a laser beam by ${90}^{\circ }$ using a single mirror?" It might have been more clear if the author had written: "How can you change the direction of a laser beam by ${90}^{\circ }$ using only one mirror?" Both questions are asking the same thing.