Transactional data can be best explained by using the Market-Basket example. A basket can contain many items that have been bought at a grocery store. For example:
basket_1 = {bread, cheese, milk}
basket_2 = {apple, eggs, salt, yogurt}
....
basket_n = {biscuits, eggs, milk}
Here are the definitions used in terms of transactional data.
An Item is an article in a basket, or an attribute-value pair
A Transaction consists of all the items purchased together in one basket. It usually has a transaction Id (TID) along with all the items.
A Transaction Dataset is a set of such transactions.