Okay, so I have a powerful imagination, apparently. I constantly visualize fictional places and people that have only ever existed in my head, and I remember places and people from 20+ years ago that were never real. I can trivially visualize characters in the real world in front of me, and what they would do. I can replay movies in my head, just as vividly as the original, any time I want. Only as well-detailed as my memory captured it, of course, but there is not really a difference in the experience for me between the first time and the replay in my head.
Visualizing for me is like having a second set of eyes that see an entirely different world from this one. It has no less detail -- it can, in fact, have even MORE detail than this world, since it isn't limited by my eyes -- and that, I think, is what people are discussing. Maybe this doesn't fit the definition of visualizing.
People in this thread are saying that when they visualize something, they don't actually see it. But for me, there have actually been brief moments where I can't tell the difference between my visualizations and the real world. I can get lost in the worlds in my head (in a figurative sense) and while my eyes are still open and are transmitting an image to my brain, and that image is present, I'm not looking at it. I'm looking at the images from another world in my head.
There is obviously a spectrum here. But my mind's eye is a window into worlds that don't exist, that can't exist. It's so much more than just a way to remember how things in the real world looked, I use mine to assemble images of things that never existed, and to see things that can never happen. And this doesn't take effort for me. I can do this all day, effortlessly.
I feel like you're going to tell me that I just have an overactive imagination and that has nothing to do with visualization, but that just doesn't make sense to me. This is what people are talking about, it has to be. If what we're talking about isn't visualizing, then it's something that we need to invent new words for, but I've got it, and it is abundantly clear that other people just don't.
If you ask me to visualize an apple, I can do that. I can also put a red delicious apple next to a braeburn, a granny smith and a honeycrisp, and I can actually see, in my mind's eye, the little white dots on the tall, dark red delicious, the bigger faint white dots on the bright green granny smith, and the streaked red and yellows on the gala. This is an actual, complete image in my mind. But it's also so ... small. It's such a tiny thing compared to what I can visualize.
Tell me the three apples are in a fruit bowl, and then tell me the bowl was thrown out the window, and I see this whole thing happen in a house, whose entire structure I can now describe from memory. It has two floors. A little kitchenette to the right. The window that was broken was left of the front door. It's in the country. There was a clothesline outside to the right of frame, trees waving in the wind in the background. Tall ones. Oaks, maybe, but I don't know how to identify trees. I just know what they look like. There was a lake in the background. I saw all of this detail in a few seconds in my mind's eye. Not with my real eyes, and it is effortless.
But the way I understand the word "see" is different. Light enters the eyes, sure, but that's not what you see. You are in your brain, and all that data from the eyes passes through the visual cortex. Your eyes can't see detail or color except near the center, so the image you actually "see" is actually a heavily-processed recreation created by your brain. The images from the real world only ever exist in your mind to begin with. They don't exist anywhere else.
Honestly, I can't tell you how I tell the images I visualize apart from the ones in the real world. I just know. But the imagery from my mind's eye is so compelling that I can honestly tell you that the only time I spend time in the real world is because I'll die if I don't.