Definition
Plain language
The rule a distributed system follows about how up-to-date and consistently ordered the data looks to everyone reading it.
As stated in the literature
A formal contract specifying which orderings of reads and writes a distributed store may expose to clients; ranges from strong models like linearizability down to weaker ones like causal consistency, monotonic reads, and read-your-writes.
Also called: consistency models
Why it matters: It defines what guarantees an app can rely on when data is spread across many machines, so choosing the wrong one leads to confusing bugs like seeing stale or out-of-order data.
For example, a strong consistency model guarantees that once you post a comment, you and everyone else immediately see it, while a weaker one might briefly show some readers the old version.