(1) The act of permeating or penetrating into a substance, cell, or tissue; said of gases, fluids, or matter held in solution. (2) The gas, fluid, or dissolved matter that has entered any substance, cell, or tissue. SYN: infiltrate (2). (3) Injection of solution into tissues, as in infiltration anesthesia. (4) Extravasation of solutions intended for intravascular injection.