Occurrence, n.
Etymology: [Cf. F. occurrence. See Occur.]
1. A coming or happening; as, the occurence of a railway collision. Voyages detain the mind by the perpetual occurrence and expectation of something new. I. Watts.
2. Any incident or event; esp., one which happens without being designed or expected; as, an unusual occurrence, or the ordinary occurrences of life. All the occurrence of my fortune. Shak.
Syn. -- See Event.
Shades of meaning
Fernald has no entry of its own for occurrence; he lists it under circumstance, event.
Where he writes about it:
An occurrence is, etymologically, that which we run against, without thought of its origin, connection or tendency. — from event