The notion of transaction costs is pervasive in each field of the economic theory. But, what is a transaction? This paper reports the meanings of transaction as proposed by Ronald Coase, Coasians such as Oliver Hart and Oliver Williamson, and a pre-Coasian like John Commons—I will advocate the latter notion of transaction. I argue that each transaction has three dimensions and, therefore, related costs (i.e. transaction costs) depend on a puzzling combination of these three dimensions.