Real Dictionary

secede
verb: If a region or group secedes from the country or larger group to which it belongs, it formally becomes a separate country or stops being a member of the larger group.

secede in American English
to withdraw formally from membership in, or association with, a group, organization, etc., esp. a political group
intransitive verb: to withdraw formally from an alliance, federation, or association, as from a political union, a religious organization, etc

secede in British English
verb: (of a person, section, etc) to make a formal withdrawal of membership, as from a political alliance, church, organization, etc