A dangerous weapon is any material object that was designed with the purpose of, or has the ability to, seriously harm or kill a person. Included in the list of dangerous weapons are guns, knives beyond a certain length (this can vary by state law but it is generally anything over 4 inches), any knife that is designed specifically for fighting such as a switchblade, or skinning knives used in hunting. Also considered among dangerous weapons is any kind of martial arts weapons like throwing stars or nunchucks.
Items such as guns and hunting knives are only illegal in circumstances where they are used to commit a crime. Other objects listed as dangerous weapons are always illegal. Any object that is used to harm a person can be labelled as dangerous in a criminal court case; if a person bludgeons another person to death with a rock, the rock would be considered the dangerous weapon.