Numerical age in geology refers to the actual age of a rock or geological feature, expressed in years. This is determined through various dating methods, such as:

* Radiometric dating: Measures the decay of radioactive isotopes within the rock.

* Fossil evidence: Comparing the fossils found in the rock to known geological time periods.

* Stratigraphy: Studying the layers of rock and their relative positions.

So, when geologists talk about a rock being "millions of years old," they are referring to its numerical age.