I could just say "height", but my reaction is unpredictable, with no logical consistency. I can't tolerate standing anywhere near the edge of a cliff or building without a railing, which I realize is fairly common; as long as there is an apparently sturdy railing, I'm not bothered at all. Being in a plane, however small, or a hot air balloon, doesn't bother me in the least. But to change a ceiling light comfortably, I need a good stepladder; if I stand on a stool even a foot off the floor, and extend my reach up for any reason, I'm very shakey. We went to the Grand Canyon a few years ago. We walked a just a little way down the Bright Angel trail. I wasn't really panicky, but it made me edgy. The next day we rode down on mules. I found myself leaning over to look down without the least discomfort.
My nephew has a boat and parasail. I'm really torn about trying the thing; I really don't know whether I would take it in stride, or stress out to the point of potential heart attack.