While I disagree with archaeological blogger, David Gill, on many points, I do agree with his limitation of the term "stolen" to:
objects stolen from a museum;
objects stolen from archaeological stores or similar holding areas;
objects stolen from a private collection;
objects stolen from a monument at a recorded archaeological site.
Unfortunately, others [including some U.S. Courts] have also used the term "stolen" to refer to artifacts removed from the ground outside recorded archaeological sites, under circumstances that may violate a source country's patrimony laws. See: http://www.archaeological.org/media/docs/AIA%20Frequently%20Asked%20Questions.doc
In addition, one suspects that some even consider artifacts to be "stolen," if they leave a source country without an export permit. See: http://culturalpropertyobserver.blogspot.com/search?q=Sisto
I suspect most collectors would also agree with Gill's definition, and share my concern about the expansion of the meaning of the term "stolen" in the cultural property debate.