This model is a slightly weaker model than strict consistency. Instead of a write being available instantaneously to all processes, the order of writes to variables by different processes has to be seen as the same on all process. The operations of each individual process should appear in this sequence, as written in the program.

Leslie Lamport mentions that, Sequential Consistency is met if the result of any execution is the same as if the operations of all the processors were executed in some sequential order, and the operations of each individual processor appear in this sequence in the order specified by its program (reference: how to make a multiprocessor computer that correctly executes multiprocess programs and computers, IEEE Transactions by Leslie Lamport).

The diagram on the left shows a sequentially consistent system (here, all processes see b written before a). On the right, however, is a system that is not sequentially consistent (P3 sees b as the value of X, while P4 sees the value as a):