Weak consistency

The name weak consistency may be used in two senses. In the first sense, strict and more popular, weak consistency is one of the consistency models used in the domain of concurrent programming (e.g. in distributed shared memory, distributed transactions etc.). 
A protocol is said to support weak consistency if:
Therefore, there can be no access to a synchronization variable if there are pending write operations. And there can not be any new read/write operation started if the system is performing any synchronization operation.
In the second, more general, sense weak consistency may be applied to any consistency model weaker than sequential consistency.
A stricter condition is strong consistency, where parallel processes can observe only one consistent state.