The definition of Official Development Assistance (ODA) has for 40 years been the global standard for measuring donor efforts in supporting development co-operation objectives. It has provided the yardstick for documenting the volume and the terms of the concessional resources provided, assessing donor performance against their aid pledges and enabling partner countries, civil society and others to hold donors to account. Yet for all its value, the ODA definition has always reflected a compromise between political expediency and statistical reality. It is based on interpretation and consensus and therefore allows for flexibility. It has evolved over the decades, while preserving the original concepts of a definition based on principal developmental motivation, official character and a degree of concessionality. While agreement on the ODA concept was a major achievement, discussion of the appropriateness of this measure has never ended. The paper documents the evolution of the ODA concept and proposes a possible new approach to measuring aid effort.