infiltration

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The act or process of infiltrating.
• noun The state of being infiltrated.
• noun Something that infiltrates.

from The Century Dictionary.

• noun In geology, the introduction of water into porous or open-textured rock. The term is specially employed in the discussion of ore-deposits to describe the introduction of new minerals in solution.
• noun The act or process of infiltrating.
• noun In pathology, a morbid condition of any portion of tissue produced by the accumulation in it of substances introduced from without: distinguished from degeneration, where the substance abnormally present is produced from the tissue itself through faulty metabolism.
• noun That which infiltrates; a fluid, or matter carried by a fluid, which enters the pores or cavities of a body.

from the GNU version of the Collaborative International Dictionary of English.

• noun The act or process of infiltrating, as of water into a porous substance, of a fluid into the cells of an organ or part of the body, or of persons into a hostile organization.
• noun The substance which has entered the pores or cavities of a body.
• noun (Med.) See under Fatty.
• noun a filter gallery.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun The act or process of infiltrating, as of water into a porous substance, or of a fluid into the cells of an organ or part of the body.
• noun The substance which has entered the pores or cavities of a body.
• noun The act of entering a physical location and/or organization secretly.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the slow passage of a liquid through a filtering medium
• noun a process in which individuals (or small groups) penetrate an area (especially the military penetration of enemy positions without detection)