A reunion usually refers to a gathering of folks who haven't seen each other for a long time. High school reunions are notorious for reuniting middle aged people who are anxious to relive their high school years.
Since a union is a state of togetherness — like a marriage — a reunion is when people get back together after some sort of separation. This could refer to a political reunification — like the reunion of East and West Germany as one nation — but it's usually smaller in scale. At high school and college reunions, people see friends they haven’t seen for years. A reunion doesn't have to be so official. Seeing any friend after an absence is a reunion.