Final answer:
The shortest unit of geologic time is the epoch, which is a subdivision of a period, and is shorter than a period, era, or eon.
Step-by-step explanation:
When it comes to understanding the Geologic Time Scale, it's clear that there are different units used to measure periods of Earth's history. These units include eons, eras, periods, epochs, and ages. Among these, the shortest unit of geologic time is the epoch. Eons are the largest division and are subdivided into eras, which are then subdivided into periods, which are further broken down into epochs. Periods, while significant, are longer than epochs but shorter than eras or eons. Epochs are the shortest standard span of geological time, characterized by distinctive climate conditions, types of life, and geological events.