(1) (Techn) Any undesirable foreign matter that has an adverse effect on a finished article, material, or product. (2) (Nucl Safety) The mixing of radioactive material with part of one's environment, for example, by radioactive fallout contamination of the earth. More precisely, radioactive contamination. Ant: Decontamination. (3) (Meld) Entry of undesirable organisms into some material or object. Ant: Decontamination. Note: Referring to bacteria which have fallen into a wound, but in which bacterial invasion is not yet apparent