Culet correctly pronounced 'kyoo-lit' comes from the Latin word 'culus' meaning bottom.  The culet is the facet at the very bottom of a cut gemstone although not all faceted gemstones have one.  In some, the culet is closed which means the tip comes to a sharp point.  Although a closed culet actually means there is no culet at all, many still refer to the pointed tip of a faceted gemstone as being the culet.