Short Definition
CAP theorem says a distributed data system cannot simultaneously guarantee consistency, availability, and partition tolerance during a network partition.
Intuition
When the network splits, a system must choose between answering every request and preserving one consistent view of data.
Technical Definition
Under partition, a distributed system must trade off availability against linearizable consistency; partition tolerance is a condition to survive, not an optional feature.
Example
A replicated store may reject writes during a partition to preserve consistency.
Common Misunderstandings
CAP is often oversimplified as choosing two of three forever.
Real systems make more nuanced consistency and availability tradeoffs.