Define cluster.stats()?

It is define in fpc package which provide a method for comparing the similarity of two clusters solution using different validation criteria.

Cluster sampling is used in statistics when natural groups are present in a population. The whole population is subdivided into clusters, or groups, and random samples are then collected from each group. Cluster analysis is a statistical method used to group similar objects into respective categories. It can also be referred to as segmentation analysis, taxonomy analysis, or clustering.