In engineering, entrainment is the entrapment of one substance by another substance. For example: * The entrapment of liquid droplets or solid particulates in a flowing gas, as with smoke. * The entrapment of gas bubbles or solid particulates in a flowing liquid, as with aeration. * Given two mutually insoluble liquids, the emulsion of droplets of one liquid into the other liquid, as with margarine. * Given two gases, the entrapment of one gas into the other gas. * "Air entrainment" - The intentional entrapment of air bubbles into concrete. * Entrainment defect in metallurgy, as a result of folded pockets of oxide inside the melt.