The term ionic strength is defined differently in different textbooks and monographs. Some define it in terms of mol kg-1, while others define it in terms of the non-SI unit mol L-1. The conversion to the SI unit mol m3 creates a problem in computing mean ionic activity coefficients. A dimensionless ionic strength unit has also appeared. These various definitions make the concept of ionic strength very confusing to students, especially in the use of this quantity to calculate mean activity coefficients through the Debye-Hückel theory. To resolve this confusion, a recommendation is given to define ionic strength as a dimensionless quantity.