Consistency : all nodes see the same data at the same time
Availability : node failures do not prevent survivors from continuing to operate
Partition Tolerance : the system continues to operate despite arbitrary message loss

Partition tolerance refers to the ability for a system to continue to operate in the presence of a network partitions.  For example, if I have a database running on 80 nodes across 2 racks and the interconnect between the racks is lost, my database is now partitioned.  If the system is tolerant of it, then the database will still be able to perform read and write operations while partitioned.  If not, often times the cluster is completely unusable or is read-only.
모, 대충 이해하기로 네트웍의 문제로 인해서 정말로 서비스하는 노드가 분리(partition)되어서 잘 동작(tolerance)을 해야 된다는 얘기인듯..