invasion

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The act of invading, especially the entrance of an armed force into a territory to conquer.
• noun The entry into bodily tissue and subsequent proliferation of an injurious entity, such as a pathogen or tumor.
• noun An intrusion or encroachment.

from The Century Dictionary.

• noun In phytogeography, the phenomenon of the movement of plants from an area of one character into one of a different character, and their colonization in the latter.
• noun The act of invading a country or territory as an enemy; hostile entrance or intrusion.
• noun Hence A harmful incursion of any kind; an onset or attack, as of disease.
• noun Infringement by intrusion; encroachment by entering into or taking away what belongs to another: as, an invasion of one's retirement or rights.

from the GNU version of the Collaborative International Dictionary of English.

• noun The act of invading; the act of encroaching upon the rights or possessions of another; encroachment; trespass.
• noun A warlike or hostile entrance into the possessions or domains of another; the incursion of an army for conquest or plunder.
• noun The incoming or first attack of anything hurtful or pernicious.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A military action consisting of armed forces of one geopolitical entity entering territory controlled by another such entity, generally with the objective of conquering territory or altering the established government.
• noun figuratively The entry of a large group into a new area.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the act of invading; the act of an army that invades for conquest or plunder
• noun any entry into an area not previously occupied
• noun (pathology) the spread of pathogenic microorganisms or malignant cells to new sites in the body