invasion

(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. Synonyms: encroachment, intrusion. Examples: "An invasion of tourists." "An invasion of locusts."

(noun) (pathology) the spread of pathogenic microorganisms or malignant cells to new sites in the body. Example: "The tumor's invasion of surrounding structures."