secession /sɪˈsɛʃən/ TEM8

• 1. 

  N-UNCOUNT The secession of a region or group from the country or larger group to which it belongs is the action of formally becoming separate. (从国家或大集团中正式的)脱离

  例：

  ...the Ukraine's secession from the Soviet Union.

  ...乌克兰之退出苏联。