An addition, an extension.
"The new library wing was designed as a modern annex to the historic main building."
In plain English: An annex is an extra room or building added to make part of a larger house or structure.
"The map clearly shows where the country annexed territory was located before the war ended."
Usage: As a noun, annex refers specifically to a building added onto or connected with the main structure of a larger one. Use this term for physical extensions like wings on a museum rather than metaphorical additions such as new clauses in a contract.
To add something to another thing, especially territory; to incorporate.
"The neighboring country annexed the disputed region after winning the border war."
In plain English: To annex something means to officially add it as part of your own territory or group.
"The country annexed the neighboring territory after winning the war."
A census-designated place in Malheur County, Oregon, United States.
"Residents of Annex often travel to Vale for their weekly grocery shopping."
The word annex comes from the French annexe, which was borrowed from the Latin annexus. Originally, it meant something attached or joined to a main structure.