Nodes, comments, taxonomy terms, and users, using the terminology used since Drupal 7, are generally called "entities."
"Entities" is the term that in Drupal 7 is used for nodes, comments, taxonomy terms, and users. Third-party modules can then define new entity types.
One of the features of Drupal 7 entities is that they can have associated some fields created through the field UI, or implemented by third-party modules.
In Drupal 8, the API to handle entities has been extended, and entities are now object of specific classes implementing the interface; support for saving entities in the database has been added, where in Drupal 7 saving an entity was done by procedural functions.
- An Introduction to Entities
- Working with content types and fields
- [meta] Drupal 8 Entity API improvements