Role

Definition

From the Basic Formal Ontology (BFO): A role is: A realizable entity [snap:RealizableEntity], the manifestation of which brings about some result or end that is not essential to a continuant [snap:Continuant] in virtue of the kind of thing that it is, but that can be served or participated in by that kind of continuant [snap:Continuant] in some kinds of natural, social or institutional contexts.

Comments

All BFO concepts can be explored here: http://jowl.ontologyonline.org/bfo.html

Basically, a role pretty much means what you think it means. If you can stop doing it without stopping being yourself, it's a role. You can play the role of president, or person who is sitting in the corner. Being a two-legged mammal is not a role, however (and don't get started on what happens if you have to have a leg amputated).