I was wondering about the reason of the choose of the X as symbol used to represents the "Close application" operation.
In the article "The origins of the use of [x] in UI design" you can read:
The use of [x] for close and [o] for open could come from the Japanese symbols batsu and maru.
Batsu (x) is the symbol for incorrect, and can represent false, bad, wrong or attack, while maru (o) means correct, true, good, whole, or something precious. Batsu and maru are also common hand gestures. Cross your arms over your chest for batsu, circle your arms over your head for maru.