What power of 10 does the prefix "micro" refer to?

The prefix "micro" refers to 10 to the negative-sixth power. That is the equivalent of one in a million. For example, 1 micrometer, also called a micron, is 1 millionth of a meter. The lowercase version of the 12th Greek letter, mu, is used to represent micro.

"Micro" also means small; many microorganisms are small, but not all are exactly 1 micron. Dust mites, for example, range from 0.1 to 0.3 microns in size.

An average human hair is 75 microns across. Human eyes can usually see particles that are about 25 microns across; anything smaller than that requires a microscope.