Things that are immaterial have no physical form (like a ghost) or are unimportant (like most ghost stories).
Something that’s material has substance, right? You can touch it or it's important. So the opposite is the word immaterial, which means something that doesn't matter, or has no physical substance, or which adds nothing to the subject at hand. This word is heard a lot in court, where "It's immaterial!" dismisses evidence or testimony as meaningless or beside the point. You could say the wind is literally immaterial, though windiness is not immaterial if you’re going kiting.