WordNet sense 1 (a small sharp-pointed tip resembling a spike on a stem or leaf):

pricker, prickle, spine, thorn, spikelet

WordNet sense 2 (an adhesive label):

paster, gummed label

WordNet sense 3 (a particularly difficult or baffling question or problem):

poser, toughie, stumper

WordNet sense 4 (a short knife with a pointed blade used for piercing or stabbing):

dagger