The scientists are using eon, era, and period.
In order to manage to divide the geological time scale more practically the scientists used a simple type of hierarchy after they were able to agree when a certain line should be drawn. The eon is the largest unit, it contains few eras in it, while each era is divided into multiple periods, thus the eon is the largest unit, the era is the second largest, and the period is the smallest one. All of them are used for better orientation and understanding of the geological past of our planet.