A transaction is any kind of action involved in conducting business, or an interaction between people. When you go to the bank, fill out a form, and deposit your paycheck, you make a transaction.
An important business deal can be called a transaction, particularly the buying or selling of goods, but you can call any exchange with another person a transaction. There are transactions involving money, ideas, and even e-mail. The Latin root transactionem describes an agreement or accomplishment. This led to a mid-15th century version of transaction that described the adjustment of a dispute.