In economics and related disciplines, a transaction cost is a cost incurred in making an economic exchange (restated: the cost of participating in a market). For example, most people, when buying or selling a stock, must pay a commission to their broker; that commission is a transaction cost of doing the stock deal. Or consider buying a spatula from a store; to purchase the spatula, your costs will be not only the price of the spatula itself, but also the energy and effort it requires to find out which of the various spatula products you prefer, where to get them and at what price, the cost of traveling from your house to the store and back, the time waiting in line, and the effort of the paying itself; the costs above and beyond the cost of the spatula are the transaction costs. When rationally evaluating a potential transaction, it is important to consider transaction costs that might prove significant.

A number of kinds of transaction cost have come to be known by particular names:

The idea that transactions form the basis of an economic thinking was introduced by the institutional economist John R. Commons (1931). He says that,