explicit-instantiation is a template-related declaration, where a template class is explicitly instantiated for a particular type by a declaration. Example (from § 14.7.2 draft N3337)
template<class T> class Array { void mf(); };
// explicit instantiation of Array template class for char
// leads to all functions being instantiated too
template class Array<char>;
// explicit instatiation of only the member function mf for int
template void Array<int>::mf();
int a;
is a simple-declaration like Joachim mentions in comment.