Webster’s Dictionary defines “annexation” as “The act of annexing; process of attaching, adding, or appending; the act of connecting; union; as, the annexation of Texas to the United States, or of chattels to the freehold.”
(The language is a bit outdated in this particular definition with the use of “chattels” and “freehold” - terms and subjects with which students today may not be familiar - so focus on the example of Texas being annexed to the United States in 1845 following war with Mexico.)
Since the definition uses the word “annexing”, it is also necessary to look up its base verb, “annex”, which is defined by the Merriam-Webster Dictionary in this way: “To add (an area or region) to a country, state, etc. : to take control of (a territory or place).”
The answer to the question, therefore, should be something similar to the following:
“Annexation is the act of annexing, or adding, an area or region to the territory of an existing national or political unit. A good example would be when Texas, a region of Mexico, was annexed to the United States, a different national unit, in 1845."