(1) The beginning or incursion of a disease. (2) Local spread of a malignant neoplasm by infiltration or destruction of adjacent tissue; for epithelial neoplasms, invasion signifies infiltration beneath the epithelial basement membrane. (3) Entrance of foreign cells into a tissue, such as polymorphonuclear leukocytes in inflammation. [L. invasio, fr. in-vado, pp. -vasus, to go into, attack]
