What is eventual consistency and when is it acceptable?

Eventual consistency means all replicas will eventually converge to the same value, but may temporarily show different values. Trade-off for availability and partition tolerance (CAP theorem). Acceptable when: stale reads are tolerable (social media feeds), high availability more important than immediate consistency, system can handle temporary inconsistency. Not acceptable: financial transactions, inventory (overselling), security-critical data. Design with conflict resolution (last-write-wins, merge). Common in distributed databases.