The flow of a liquid through pores, pinholes or holes resulting from imperfections or degradations of a fabric. In a filtration context, a measure, in percent, of particles of a given size that pass through a filter. If no particles pass through, then 100% were trapped. If 97% of particles are trapped, then penetration is 3%(100% – 97% = 3%) and the filter is 97% efficient. Penetration is used to measure the performance of high efficiency filters.