What is a single mirror?

Textbooks and other technical reference manuals frequently contain a lot of technical terms and jargon, which the authors haven't taken the time to carefully define. Knowing what these terms mean is essential to understanding the content.

The word "single" here refers to the presence of just one mirror; the author is not talking about some unique kind of mirror.

It might have been clearer if the author had written, "How can you change the direction of a laser beam by 90º using only one mirror?" Both questions ask the same thing. If this term had been used in a textbook question, it might have said something like this: "How can you change the direction of a laser beam by 90º using a single mirror?"

A single mirror refers to a reflective surface that is not divided or layered, providing a singular reflective interface.