Dating activity with more than one person, or while being "married" to another. PolyDating is a subset of PolyAmory. A primary component of this is that all parties are aware of all other parties. This is distinct from "Cheating" as that activity requires deception and concealment of other relationships.
Jeff and Sally both have partners outside their relationship with the full knowledge and consent of their "primary" partner. PolyDating can occur without any "primary" relationship, as in the case of "not going steady" or "free to see other people."