What does the define function do in AngularJS?

Its probably because you're assuming its an Angular function. I believe this is RequireJS.

http://requirejs.org/docs/api.html#define

That is not AngularJS function but RequireJS module definition.

More about how can you use RequireJS AMD library with AngularJS - read here: http://www.sitepoint.com/using-requirejs-angularjs-applications/