Just for clarification, in ORACLE you see the term container, too. However, for them, it is a file which could be one of many similar files. The container file "contains" whatever you put there, which could include references to other containers. At one point at our Navy site, we had an ORACLE instance with 20 containers spread over 12 different disks.
In that context, your GUI or command-line can be pointed to a container file to enumerate what it contains. The Navigation pane is conceptually similar, particularly if you remember that external references are shown specially in the Nav pane. If you have a split DB with an FE/BE situation, you can individually open the files and will see different things - because for example, the FE contains links to the BE for its tables and such, but the BE does NOT contain references to the forms, reports, macros, and modules of the FE.
Therefore, when you see "database container" - it COULD be something that was written by someone accustomed to other database products such as ORACLE. The NAV pane is a more modern nomenclature but it still is merely a "window into a container" if you think about what you see in FE/BE situations.