Final answer:
A geochronologic division is a division of geologic time based on numerical ages determined through radiometric dating, while a chronostratigraphic division is a division of geologic time based on the relative age of rocks and the events that have occurred within them.
Step-by-step explanation:
A geochronologic division, also known as a time unit, is a division of geologic time based on numerical ages determined through radiometric dating. These divisions are used to represent specific periods of time in Earth's history. For example, the Paleozoic era is a geochronologic division that represents a specific time period in Earth's history.
A chronostratigraphic division, or time-rock unit, is a division of geologic time based on the relative age of rocks and the events that have occurred within them. These divisions are based on the layering and succession of rocks. A chronostratigraphic division represents a specific section of rock layers, such as a formation or a bed, that can be correlated across different regions. For example, the Ordovician period is a chronostratigraphic division that represents a specific section of rock layers that were deposited during that time period.