Clincher

Noun

 * 1) That which clinches; that which makes something final or firm.

Thesaurus
conclusive argument, coup de grace, crusher, death stroke, deathblow, end-all, ender, final stroke, finisher, finishing stroke, floorer, kayo, kayo punch, knockdown argument, knockout, knockout blow, last dab, quietus, settler, sockdolager, stopper, trump