The span of geologic time that is subdivided into periods is called an Era.

Here's a breakdown:

* Eons: The largest division of geologic time. For example, the Phanerozoic Eon (the current eon).

* Eras: Subdivisions within an eon. For example, the Paleozoic Era, Mesozoic Era, and Cenozoic Era within the Phanerozoic Eon.

* Periods: Subdivisions within an era. For example, the Permian Period within the Paleozoic Era, or the Cretaceous Period within the Mesozoic Era.

* Epochs: Subdivisions within a period. For example, the Pleistocene Epoch within the Quaternary Period.

So, to summarize: Eras are the subdivisions of geologic time that contain multiple periods.