[n] a point or fact or remark that settles something conclusively [n] an argument that is conclusive
decisive factor, determiner, determining factor
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, KO, last dab, quietus, settler, sockdolager, stopper, t