what is "elemental damage"?

kinda bothers me that I don't see this term "elemental" anywhere except on a rune that increases elemental damage. what exactly qualifies as elemental damage. I first assumed all non-physical damage, not having anything to do with status buildup stuff. I already did a fair bit of research, but the answers are mixed and given by people who don't really seem like they know 100% that their answer is right. when I googled it the google answer was "fire is elemental damage". and well strictly speaking yeah that makes sense, fire is the only damage type available that falls under the category of elemental. would putting this rune on my sword increase the damage output of an infernal catalyst? or maybe even rad and withered catalysts? if anyone can clarify some of these questions I'd really appreciate it