What is Annexation?

Legal Definition
Annexation (Latin ad, to, and nexus, joining) is the political transition of land from the control of one entity to another. It is also the incorporation of unclaimed land into a state's sovereignty, which is in most cases legitimate. In international law it is the forcible transition of one state's territory by another state or the legal process by which a city acquires land. Usually, it is implied that the territory and population being annexed is the smaller, more peripheral, and weaker of the two merging entities, barring physical size. It can also imply a certain measure of coercion, expansionism or unilateralism on the part of the stronger of the merging entities. Because of this, more positive euphemisms like political union/unification or reunification are sometimes seen in discourse. Annexation differs from cession and amalgamation, because unlike cession where territory is given or sold through treaty, or amalgamation (where the authorities of both sides are asked if they agree with the merge), annexation is a unilateral act where territory is seized and held by one state and legitimized via general recognition by the other international bodies (i.e. countries and intergovernmental organisations).

During World War II, the use of annexation deprived whole populations of the safeguards provided by international laws governing military occupations. The authors of the Fourth Geneva Convention made a point of "giving these rules an absolute character", thus making it much more difficult for a state to bypass international law through the use of annexation.
-- Wikipedia
Legal Definition
Property. The union of one thing to another.

2. In the law relating to fixtures, (q. v.) 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. Shep. Touch. 469. Vide Anios & Fer. on Fixtures, 2.

3. 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 larch 1, 1845., See Texas.
-- Bouviers Law Dictionary
Legal Definition
The act of attaching, adding, joining or uniting one thing to another; generally spoken of the connection of a smaller or subordinate thing with a larger or principal thing. The attaching an illustrative or auxiliary document to a deposition, pleading, deed, etc., is called "annexing" it. So the incorporation of newly-acquired territory into the national domain, as au integral part thercof, is called "annexation," as in the case of the addition of Texas to the United States.

In the law relating to fixtures: Actual annexation includes every movement by which a chattel can be joined or united to the frcehold. Constructive annexation is the union of such things as have been holden parcel of the realty, but which are not actually annexed, fixed, or fastened to the tree-hold. Shep. Touch. 469 ; Amos & F. Fixt. 2. In Scotch law. The union of lands to the crown, and declaring them inalienable. Also the appropriation of the church-lands by the crown, and the union of lands lying at a distance from the parish church to which they belong, to the church of another parish to which they are contiguous.
-- Black's Law Dictionary
Legal Definition
The affixing of chattels to land in such manner that they become in law a part of it. See. 14 Cal. 64.
-- Ballentine's Law Dictionary