# Universe description

A Universe describes units of a Unit Type that meet shared defining characteristics, independently of time and geography. For example, "women aged 18 or older" narrows the Unit Type "persons" without specifying a place or reference period. This follows the Universe level of the [DDI Common Core, version 1](https://doi.org/10.5281/zenodo.17297309) Unit Cascade (p. 5).

A Universe may be organized into hierarchical sub-universes. Alongside its name, label, and description, it can identify a defining concept, provide generation code describing how it is defined or derived from another Universe, and indicate whether its definition uses inclusion or exclusion criteria.

For new descriptions, use Population to add both geographic and temporal scope to a Universe. Universe retains `LocationValueReference`, `GeographyOfUniverse`, and `TimePeriod` for compatibility with existing metadata, but these properties are deprecated. Record geography in Population's `GeographyOfPopulation` and time in `TimePeriodOfPopulation`.