Supererogation: Supererogation is the technical term for the class of actions that go “beyond the call of duty.” Roughly speaking, supererogatory acts are morally good although not (strictly) required. [...] The Latin etymology of “supererogation” is paying out more than is due (super-erogare), and the term first appears in the Latin version of the New Testament in the parable of the Good Samaritan. Although we often believe that Good Samaritanism is praiseworthy and non-obligatory at the same time, philosophical reflection raises the question whether there can be any morally good actions that are not morally required, and even if there are such actions, how come they are optional or supererogatory. (more here) 1
1. Heyd, David, "Supererogation", The Stanford Encyclopedia of Philosophy (Fall 2008 Edition), Edward N. Zalta (ed.).