Short Definition
Cache coherence is the property that multiple caches maintain a consistent view of shared data.
Intuition
If two processors cache the same value, the system needs rules so they do not silently disagree after updates.
Technical Definition
Cache coherence protocols coordinate reads and writes across caches so memory operations observe a consistent ordering for shared locations.
Example
A multicore CPU uses coherence protocols so one core sees updates made by another core.
Common Misunderstandings
Cache coherence is not the same as full memory consistency.
Coherence solves hardware-level sharing problems, but software synchronization is still needed.