Refers to an unexpected meeting between people that was not planned. Running into my high school teacher at the airport was completely coincidental.
Pro Tip 2/3
Mistaken Causation
Two events may occur simultaneously without one causing the other. Her success and my resignation were purely coincidental and unrelated.
Pro Tip 3/3
Surprising Parallel
Used when two things happen together in a way that seems surprising or noteworthy. It was quite coincidental that we both wore the same dress on the same day.