In U.S. usage, "reunion" is not a synonym for "meeting." A "reunion" has a very specific meaning, and is typically used in these three contexts: family reunion, high school reunion, and college reunion. A reunion is a large organized gathering of dozens to hundreds of people, usually people who have been together at some time in the past, and often as a regular anniversary event. For example,
Dear Classmates
The 25th Reunion of the St. Ruan High School Class of 1989 will be held October 25th and 26th at the Roysburg Country Club. The official hotel will be the Roysburg Best Western, with a special room rate for those who book before September 25th. The Saturday event will feature a cocktail hour and light dinner, while the Sunday event will include a full dinner and ballroom dance to 1980s music, with Rob Johnson of the Class of 1989 as DJ.