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.
Stedman’s Medical Dictionary © Wolters Kluwer Health. All rights reserved.