1 [uncountable and countable]
when two things happen at the same time, in the same place, or to the same people in a way that seems surprising or unusual [↪ coincide, coincidental]:
'I'm going to Appleby tomorrow.' ' What a coincidence!I'm going up there too.'
By coincidence, John and I both ended up at Yale.
sheer/pure coincidence (=completely by chance)
I didn't set out to find you - it's sheer coincidence that I should walk along the same street.
not a coincidence/more than coincidence
After the fourth attack on her car she was convinced that the vandalism was more than just coincidence.
It was a happy coincidence that Robert arrived at the same time.
2 [singular] formal
when two ideas, opinions etc are the same
a coincidence of interest between the mining companies and certain politicians