The Buddhist technical term was first translated as ‘mindfulness’ by T.W. Rhys Davids in 1881. Since then various authors, including Rhys Davids, have attempted definitions of what precisely is meant by mindfulness. Initially these were based on readings and interpretations of ancient Buddhist texts. Beginning in the 1950s some definitions of mindfulness became more informed by the actual practice of meditation. In particular, Nyanaponika's definition appears to have had significant influence on the definition of mindfulness adopted by those who developed MBSR and MBCT. Turning to the various aspects of mindfulness brought out in traditional Theravāda definitions, several of those highlighted are not initially apparent in the definitions current in the context of MBSR and MBCT. Moreover, the MBSR and MBCT notion of mindfulness as ‘non-judgmental’ needs careful consideration from a traditional Buddhist perspective. Nevertheless, the difference in emphasis apparent in the theoretical definitions of mindfulness may not be so significant in the actual clinical application of mindfulness techniques.