If you start with the fact that OSPF uses a topology table and learns all of the routes that are inside the OSPF environment. What does the default route do? It points to prefixes that are OUTSIDE of the OSPF environment. Outside is external. So the OSPF default route is an external route.
Assuming you're referring the the "special" way of injecting a default route into OSPF, consider it's really being injected like other "foreign" routes, which also become external routes to OSPF which are all not from some OSPF link in the OSPF AS.
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