happenstance /ˈhæpənˌstæns/

• 1. 

  N-UNCOUNT If you say that something happened by happenstance, you mean that it happened because of certain circumstances, although it was not planned by anyone. 偶然情况; 巧合 [书面] [also 'a' N, oft 'by' N]

  例：

  I came to live at the farm by happenstance.

  我到这个农场来生活纯属偶然。