The Exact Definition of Open End Activity

Dears:

A discussing was going on in regard of exact definition of open end activity.

Some said that open end activity, the activity without successor and others said that open end activity which is belong and come under open end track or path even the activity has predecessors or successor but the end of track where it belongs is opened with no links to project completion date.

So now my dear I need some expertise opinions.

Thanks.