The union of one thing to another.
In the law relating to fixtures, annexation is actual or constructive. By actual annexation is understood every movement by which a chattel can be joined or united to the freehold. By constructive annexation is understood the union of such things as have been holden parcel of the realty, but which are not actually annexed, fixed, or fastened to the freehold; for example, deeds, or chattels, which relate to the title of the inheritance.
This term has been applied to the union of one country, to another; as Texas was annexed to the United States by the joint reolution of Congress of March 1, 1845.