All 3 Uses

coincidence
in
The Guardian, by Nicholas Sparks
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As if he honestly believed she'd think this whole thing were a coincidence.†
Chpt 26 *

coincidence = a situation where things happened at the same time by chance even though it was unlikely
Definitely too coincidental, she decided, especially after learning what she had earlier this morning about his past.†
Chpt 31

coincidental = a situation where unlikely things happened by chance rather than being related or arranged
You don't think all this is a little too coincidental?†
Chpt 33

Definitions:

(1) (coincidence)
a situation where two things happened at the same time or in the same way by chance even though it was unlikely
(2) (meaning too rare to warrant focus)
Much more rarely coincidence can refer to things coinciding (occurring or operating at the same place or time). For example:
- the coincidence of a target and cross hairs
- the coincidence of entangled particles in quantum physics