A sword is a weapon with a long, sharp, metal blade. If you want to dress up as a knight for Halloween, you'll definitely need a sword — and maybe a suit of armor.
Swords evolved from daggers, which, like knives, have two sharp edges instead of just one. Some time during the Bronze Age, daggers became longer and longer, eventually being classified as swords. These long, sharp weapons were used by soldiers on battlefields, and for self-defense and dueling. Sword can be traced back to a Proto-Indo-European root meaning "to cut or pierce."