The crucial distinction as far as the OP is using the term seems to be that such a datastore does not require the user's hosting provider to give them an instance of any of the common DB servers, which typically needs a more expensive plan than the usual baseline of "disk space and an httpd [that supports calling out to one or two scripting languages]".

(Basically it should run on anything more complex than a static site host. Though it's also true that sqlite would probably be a cleaner choice for the datastore without losing any of that benefit.)

Hosting wise it isn't