Everyone agrees menu links should be entities. Jury is out on Menus => Let us do menu links to start with and that will inform subsequent thought. We can, nevertheless, say that the original idea of menus as bundles of menu links is off the table.
I believe as fago is currently using the terms:
Property: A compound data element on an Entity (basically what we call "Fields" in Drupal 7).
Field: A special case of a Property that is user-added rather than defined by the module that declares the Entity type.