International Association for the Study of Pain (IASP) assigned a committee for setting a definition of pain, in year 1979. Despite many criticisms and suggestions, the definition introduced in year 1979 has been kept almost unchanged, except for the slight changes made in years 1986, 1994, and 2011. Since the criticisms from various philosophical and scientific disciplines increased in the recent period, the association assigned a team of 14 researchers and clinicians, who were at the forefront in their disciplines, in year 2018 to revise the definition. Slight modifications were suggested for the definition and explanation in year 2020. In this paper, the definition of pain suggested in year 2011 is examined first. Then, the alternative definitions and criticisms from the scientific and philosophical actors are discussed and, finally, the criticisms addressing the definition proposed in 2020 are reviewed after assessing the definition.