1853 in a political sense, "remove (a place) from colonial status," American English, from de- + colonization. Earlier as a medical term (from colon (n.2)).
The great occupation of the nations of western Europe, from the beginning of the fifteenth century to near the close of the eighteenth century, was colonization and the establishment of empire on the American continent. The year 1775 witnessed the opening of the first act in the great drama of the decolonization of this continent, the end of which is not yet. [Speech of Hon. W.H. Seward of New York in the Senate, February 8, 1853, in Appendix to the Congressional Globe, 2nd Session, 32nd Congress]