A coincidence was when two or more events would happen at the same time, but there was no clear reason why they happened together. Coincidences could be unexpected or unlikely.
In 2259, Christopher Pike noted in his captain's log that "I am left to wonder... who made the comet? How many more like it are out there? Was it coincidence that fostered the chance for more life on Persephone III? Or something more?" (SNW: "Strange New Worlds")