invasionn.[L. invasio: cf. F. invasion. See Invade.]()1. The act of invading; the act of encroaching upon the rights or possessions of another; encroachment; trespass.()2. A warlike or hostile entrance into the possessions or domains of another; the incursion of an army for conquest or plunder.()3. The incoming or first attack of anything hurtful or pernicious; as, the invasion of a disease.()()