A cluster is a collection of clinically related measures. A provider will avoid the PQRS penalty if he or she reports all of his or her Medicare patients for all of the measures contained within a cluster. The link for 2016 clusters and how it relates to the MAV can be found here: MAV.
MAV is the Measures Applicability Validation process. A clinical relation/domain test is applied when a provider submits less than 9 measures across 3 NQS domains or 9 measures across less than 3 domains. The MAV is used to determine whether or not a provider could have reported additional measures. When successfully reporting using a “cluster” of clinically related measures and a cross-cutting measure, a provider should pass the MAV.