I suppose it's a head-butt if you butt someone's head. I just looked up the incident in the 2006 World Cup Final in which Zidane "head-butted" Materazzi (after much earlier needling, Materazzi had pulled Zidane's shirt. Zidane told him that he'd give him his shirt later, to which Materazzi replied that he'd rather have his sister, hence the butt). However, the butt was to Materazzi's chest, in spite of which most reports stated that it had been a head-butt. So you may have a point there, Doug.