Made famous by Eric Brewer at UC Berkeley, this is a theorem of distributed computing that can be simplified to state that one can only practically build a distributed system exhibiting any two of the three desirable characteristics of distributed systems, which are: Consistency, Availability and Partition-tolerance (abbreviated to CAP). The theorem effectively stresses on the unreliability of networks and the effect this unreliability has on predictable behavior and high availability of dependent systems.
Infinispan has traditionally been biased towards Consistency and Availability, sacrificing Partition-tolerance. However, Infinispan does have a Partition-tolerant, eventually-consistent mode in the pipeline. This optional mode of operation will allow users to tune the degree of consistency they expect from their data, sacrificing partition-tolerance for this added consistency.
The theorem is well-discussed online, with many good resources to follow up on, including:
A more recent article by Eric Brewer himself appears on InfoQ, a modern analysis of the theorem.