Remove a base field from a custom content entity in Drupal 8
Drupal 8 comes with a new paradigm for the entity fields on content entities (there is also whole new thing of config entities, but this is not within the scope of this post). As with Drupal 7 there were properties and fields for an entity, now everything is a field (base fields, and field api fields via field UI). Similar to entity properties in D7, there are now base fields used in D8.