Prominent scholars have complained of inadequate clarity and agreement on what transactions are, and how their costs are measured. This two-part article explores this topic and suggests an alternative approach. This part examines different meanings of transaction cost used by leading scholars in this area, including John R. Commons, Ronald H. Coase, Oliver E. Williamson, Douglas C. North, Douglas W. Allen, and Yoram Barzel. It reveals prominent usages of the term that differ in several important respects. A sharper approach might focus on legal contracts and exchanges of legal titles, as suggested by Harold Demsetz. That option is explored further in Part II.