Validation Set:
A validation set is a portion of a data set used in data mining to assess the performance of prediction or classification models that have been fit on a separate portion of the same data set (the training set ). Typically both the training and validation set are randomly selected, and the validation set is used as a more objective measure of the performance of various models that have been fit to the the training data (and whose performance with the training set is therefore not likely to be a good guide to their performance with data that they were not fit to).
See also this chapter in XLMiner help ,