(gen) The taking in, incorporation or reception of gases, liquids, light or heat. (phys/chem.) Penetration is a measure, in percent, of the material passing through a filter. Mathematically, penetration is 100 minus Efficiency (percent). If a filter is 98% efficient, its penetration is 2% (1 00 - 98). Penetration is used to measure the performance of very high efficiency filters.... of one substance into the inner structure of another (cf. Retention of gas, liquid or solid on a surface due to positive interaction (attraction) between the surface and the molecules of the adsorbed material. ..., in which one substance is attracted and held on the surface of another). Occurs between a gas or vapor and a liquid. (pharm.) The process of movement of a drug from the site of application into the extracellular compartment of the body.