CAP Theorem
also called Brewer's Theorem
During a network partition a distributed system must choose between consistency and availability; it cannot have both.
The theorem is narrower than the way it is usually quoted. It says nothing about normal operation. It says that when a partition occurs, a system either refuses some requests to keep every response consistent (CP) or answers them from possibly stale state to stay available (AP).
Partitions are not optional — they are a property of networks, not a design choice — so the real statement is: pick what happens during a partition, because you will get one.
The more useful successor is PACELC: during a Partition, choose Availability or Consistency; Else, in normal operation, choose Latency or Consistency. That second half is where most systems actually live, and it is the half CAP ignores.