The word is used to refer to a thing which is meant for only some people. For example: theoretical physics is a subject that is meant only for some people. So the word can be used here instead of "meant for only some people". (The word starts with "p" I feel").
The original, very first version of this question:
A happy coincidence The word is used to refer to a thing which you happen to come along , you are not looking for it ...but in this case "co-incidence" doesn't seem right 'because only one event is happening which is favourable to the other event (without deliberate effort or thought),there is no "co-incidence".The events are not coincident(they don't occur at same time).