An Etymological Dictionary of Astronomy and Astrophysics

1. Something by which a person is bound or obliged to do certain things, and which arises out of a sense of duty or results from custom, law, etc.
   

2. Something that is done or is to be done for such reasons (Dictionary.com).

Etymology (EN): M.E. obligacioun, from O.Fr. obligacion “obligation, duty, responsibility,” from L. obligationem “an engaging or pledging,” literally “a binding,” noun of action from p.p. stem of obligare “to bind, bind up, bandage,” from → ob- “to” + ligare “to bind,” from PIE root *leig- “to bind.”

Etymology (PE): Fariz, from Mid.Pers. frêz “obligation; duty.”