While I agree that "scripting language" is a bit loosely defined (though nothing like "object-oriented"), Lua was written with a "hard and soft layers" (http://c2.com/cgi/wiki?AlternateHardAndSoftLayers) approach in mind. To me, that seems like the defining characteristic of a scripting language.

I'm not saying Lua isn't a full programming language - it's actually my language of choice for day-to-day hacking, supplemented with C as necessary. It was clearly designed to accommodate projects where it isn't primary language, though.