A simplified consistency heuristic that states that all nodes in a system are guaranteed to reach a consistent state after an infinite amount of time.
An oversimplification?
Unfortunately, eventual consistency is not very useful in practice, as a system that becomes consistent after a long period of time (say, hours or days) is not useful for real-life applications.
In general, data or state that is updated asynchronously will have an eventual consistency guarantee. (See asynchronous vs. synchronous replication.)