Learner's definition of WEAPON
1 : something (such as a gun, knife, club, or bomb) that is used for fighting or attacking someone or for defending yourself when someone is attacking you
assault with a deadly weapon
a concealed weapon
chemical/biological weapons [=weapons that use dangerous chemicals, germs, etc.]
weapons of mass destruction [=weapons that can destroy entire cities, regions, etc.]
The police never found the murder weapon. [=the weapon used to commit murder]
2 : something (such as a skill, idea, or tool) that is used to win a contest or achieve something
The pitcher's slider is his most effective weapon.
a new weapon in the fight against cancer
The mayor's campaign unleashed its secret weapon.