People love to talk about strange coincidences––like you and your mother having the same birthday, or two unrelated families named "The Martins" living next door to each other. A coincidence is something that's not planned or arranged but seems like it is.
Technically, a coincidence is an occurrence of events that happen at the same time by total accident––like you and a kid from your class at school both visit the Grand Canyon on the same day. Weird. If some guy is stalking you, always running into you wherever you go, this is not a coincidence. That guy has memorized your schedule.