剣【けん】
sword (esp. a large, double-edged one), blade, bayonet, swordsmanship, stinger, ovipositor, dart
剣道【けんどう】
kendo, Japanese martial art using bamboo swords
真剣【しんけん】
serious, earnest, real sword (as opposed to a wooden practice weapon)
刀剣【とうけん】
sword, dagger, knife, bayonet
剣劇【けんげき】
sword play, samurai drama