An itemset is a set of items. E.g. - {milk, eggs, bread} is an itemset.
A k-itemset is an itemset with exactly k items in it. The above example is a 3-itemset.
Given a transactional dataset D, the number of times that all the items in an itemset occur in D is called as the count (frequency) of that itemset.
An association rule is about the relationship between two disjoint itemsets, X and Y. It is denoted as X Þ Y. It presents the pattern → When X occurs, Y also occurs.