Available Definitions:
1) n. - The act of invading; the act of encroaching upon the rights or possessions of another; encroachment; trespass.
2) n. - A warlike or hostile entrance into the possessions or domains of another; the incursion of an army for conquest or plunder.
3) n. - The incoming or first attack of anything hurtful or pernicious; as, the invasion of a disease.