A material that has a strong chemical affinity for other materials. Such substances can be used to remove undesirable elements or compounds that are present in small quantities in an environment. Uses include the introduction of barium, often in filament form, into a sealed vacuum system in order to remove residual gases, and the introduction of phosphorus into oxide layers on silicon in order to remove mobile impurities such as sodium. The latter application is particularly important for the stabilization of MOSFETs. The process of using a getter is known as gettering.