There are legal definitions, but I usually use my own:
A dagger is just a type of knife. Typically straight and double-edged, but not always. Typically used exclusively for stabbing rather than cutting (so usage is the most obvious difference).
The definition of sword versus knife, I usually do by blade length. Anything up to around a foot is a knife, anything up to around two feet is a short sword, anything bigger is a sword. Like a dagger, a saber is just a kind of sword as far as I'm concerned.
A dagger is just a type of knife. Typically straight and double-edged, but not always. Typically used exclusively for stabbing rather than cutting (so usage is the most obvious difference).
The definition of sword versus knife, I usually do by blade length. Anything up to around a foot is a knife, anything up to around two feet is a short sword, anything bigger is a sword. Like a dagger, a saber is just a kind of sword as far as I'm concerned.