CAP Theorem is a design principle/ tool that is used to design a Distributed Systems. It states that Distributed systems cannot achieve Consistency, Availability, and Partition Tolerance simultaneously.
Consistency: At any given time, all nodes in the network must return the same, recent value.
Availability: Every working node in the network must return a response to a request within a specific amount of time.
Partition Tolerance: The network continues to operate, even if the nodes are partitioned and cannot communicate with each other for an arbitrary length of time.