A portion of code that accesses shared resources and must not be executed by more than one thread at a time to prevent data corruption or race conditions.
Borrowed from nuclear engineering where 'critical' meant a dangerous threshold requiring careful control. Computer science adopted this in the 1960s when concurrent programming emerged, emphasizing the dangerous nature of unsynchronized access.
It's like a bathroom with only one key - only one person can use it at a time, and everyone else must wait their turn. Without this rule, you'd have chaos and very unhappy users!
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