The definition of an offensive weapon is any article made or adapted for use for causing injury to a person or intended by the person having it with him for such use by him, or by some other person. (N.B. this definition includes a disguised knife).
It is an offence for any person who without lawful authority or reasonable excuse has with him in any public place any offensive weapon.
Where a particular knife is not deemed to be an offensive weapon, be aware that there is also a specific offence of having a bladed article in a public place. Please see Q337 for further information.