(pathology) Any undesirable substance or group of cells that has made its way into part of the body.

(transitive, intransitive) To surreptitiously penetrate, enter or gain access to.

(transitive) To cause to penetrate in this way.

(transitive, intransitive, of a liquid) To pass through something by filtration.

(transitive) To cause (a liquid) to pass through something by filtration.

(transitive, intransitive, medicine) To invade or penetrate a tissue or organ.

(transitive, military) To send (soldiers, spies, etc.) through gaps in the enemy line.

(intransitive, of an intravenous needle) To move from a vein, remaining in the body.