I don't want to start a big thing here... but "weapon" as defined in a lot of places is what can not "fit in your hand" from tip of your pinky to your thumb, regarding knives.
it is so grey as there are a lot of foldable ones that will fit, but when extended are larger, vs. the standard knife.
So, there is a knife (swiss army, small ones, etc.) that qualify as a knife that could be "used" as a weapon, and then there are weapons, which are larger bowie type knives, which have multiple purposes, but are normally considered weapons by definition, which is murky to begin with.
Any confusion yet? Cause I am.
cb