Which of the following best describes the term 'transaction'?

The term 'transaction' is defined as an individual action of buying or selling, which accurately captures the essence of the term in the context of financial activities and commerce. A transaction typically involves a specific exchange of goods, services, or assets, where value is transferred from one party to another. This definition reflects the primary function of transactions in facilitating economic activities, enabling businesses and consumers to conduct interactions that involve financial implications.

In contrast, the other options provided don't encapsulate the term effectively. A long-term agreement between partners denotes a contract rather than a singular transactional activity. A software process for managing data refers to a technical function, which does not relate directly to commercial exchanges. A summary of financial performance over time is more aligned with financial reporting than with the specific act of a transaction. Thus, defining a transaction as an individual action of buying or selling is the most precise and contextually accurate description.