What is stratum?

Dr. dig responds:
The past gradually gets buried over the years. Layer upon layer of dying vegetation, eroded soil, ruined buildings, even garbage thrown down in the street - all pile up in horizontal layers, the oldest layer at the bottom, the most recent at the top.

This process of horizontal layering is known as stratification. Each layer in the stratification is known as a stratum.

Archaeologists will label each stratum - stratum A, stratum B, stratum C, etc - in order to identify the artifacts found within it, and to keep artifacts from one stratum from getting mixed up with artifacts from other strata (strata is plural for stratum).