Best practice and documentation for working with entities in Drupal.
In core, Drupal 7 has the functions in files: /includes/entity.inc and hook_entity_* of /includes/system.api.php which provides limited functionality for Entity usage.
Developers who want to develop with Entity concept usually utilize the contributed module Entity API.
In Drupal 8 Entity API is being worked on in:
* #1346204: [meta] Drupal 8 Entity API improvements
* #1346214: [meta] Unified Entity Field API
* #1540656: [META] Entity Serialization API for web services (e.g. content staging)
* _#1234567: Entity Forms API