An emotion experienced when individuals perceive themselves to be recipients of an intentionally rendered benefit that is both valuable to the beneficiary and costly to the benefactor.
McCullough, M. E., Tsang, J. A., & Emmons, R. A. (2004). Gratitude in intermediate affective terrain: links of grateful moods to individual differences and daily emotional experience. Journal of Personality and Social Psychology, 86(2), 295-309. doi: 10.1037/0022-3522.214.171.1245.