A Fantasy Author’s Search History: What is the difference between a knife and a dagger?

While I was writing, I suddenly realized that there is probably a difference between a knife and a dagger. A dagger is symmetrical and double-edged and is definitely used as a weapon while a knife is normally only sharp on one edge and is more of a tool. Of course, I’m sure there are always exceptions to the rule. I’ve ended up writing characters having knives that are used for battle.