[foobiekr]

Perhaps so, but in the expansive definition you're using, even an in-memory binary tree qualifies as a database, which makes your point meaningless.

[Ericson2314]

I'm OK including that — tmpfs is similar, but we can easily exclude that by requiring persistence. The intentional definition doesn't need to be expansive the point of being useless!

[aforwardslash]

Nitpick: An in-memory hash table can be a filesystem :)