weap‧on S2 W2 [countable]
something that you use to fight with or attack someone with, such as a knife, bomb, or gun
The police are still looking for the murder weapon.
offensive weapon (=one that can be used to attack someone)
He was convicted of carrying an offensive weapon.
lethal/deadly weapon (=one that can kill)
an action, piece of information, piece of equipment etc that you can use to win or be successful in doing something:
a new weapon in the fight against AIDS
Right now, she felt the need of every weapon in her armoury (=weapon that she had), including surprise.