A gathering of former classmates, usually organized in even-numbered years after graduation (10-year reunions, 50-year reunions, etc.).
While the idea is supposed to be to catch up with old friends and see what they're up to, my take on the whole thing has been that a bunch of former popular kids organize a party, lose weight so they'll still look purty, then sit around drinking, bragging about themselves, and wishing they could turn back the clock so they'd be the popular kids again.
I could be wrong. I skipped mine and have no intention of ever attending one. I hated high school.