What is the best definition for this combination of word part micro and scope?

The combination of "micro" and "scope" most commonly refers to microscope.

Here's why:

* Micro: Means "small" or "tiny."

* Scope: Refers to an instrument for viewing or examining something.

Therefore, microscope is an instrument used for viewing extremely small objects, often invisible to the naked eye.

While "micro" and "scope" can be combined in other words, "microscope" is the most common and definitive combination.