"Hi" is more informal - more casual - than "Hello", but both are fairly informal. You usually use "Hi" with your friends and "Hello" with people you may meet 'out in the world'. More formally, "Good morning" or "Good afternoon" are used.
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zoltankr"casual" and "informal"Surely you have a dictionary! These terms are used to describe ordinary, everyday conversation. Nothing like a presidential speech or a ceremony to crown a king!