The broadest description of the most common definition of a<unk>  is a<unk>  that contains Lewis acidic and Lewis basic component.  This definition describes<unk> ity as a form of amphoterism.  At times it more convenient to describe the<unk>  as containing an electrophilic and nucleophilic, however, this description is arguably just a different phrasing of the first definition.  Other times its more convent to be more specific with the description of the contrasting groups. For example<unk>  hydrogenation<unk>  contain a positions which can accept and donate a proton and another which can accept and donate a hydride.  Starting from the oxidized form of the<unk> s the proton acceptor is a Lewis base and the hydride acceptor is a Lewis acid.
Many enzymes are believed to be<unk>  applying Lewis acids and Lewis bases to substrate as required for the given transformation.<unk> s have been used to facilitate a variety or reactions.  As already noted common examples include hydrogenation and transfer hydrogenation<unk> .  These<unk> s are especially good at delivering hydrogen to unsaturated polar bonds such as ketones.  In this process the oxygen atom receives a proton and the partial positive carbon alpha to the oxygen receives a hydride.  In direct hydrorgenation hydrogen is used as the reducing agent.  In transfer hydrogenation the hydrogen is is derived from a sacrificial alcohol such as isopropanol which delivers a hydrogen equivalent to the<unk> s and in the process produces acetone.  A number of these<unk> s can preform the hydrogengenation enantioselectively producing chiral alcohols.
Bifunctional catalyst
Heterocyclic compound
Designer drug
Protic solvent
Generic drug
Immunoconjugate
Neutron moderator
Diradical