Something that looks almost right but isn’t, which makes it usually more terrifying than something clearly wrong, strangely enough. For example, if you distort a human face, oor text... you feel wweird, don't you?
The uncanny valley is likely for Darwinism too! Back when there were multiple species of humans (Homo) (and we ofc now know we were the ones that survived to this day), fights occurred, and thus maybe a sense of discomfort still lingered within us to this day.