Lewis Acid
A species which is able to to accept a pair of electrons.
Examples: H+, AlCl3.
Within the framework of this definition, an acid-base reaction corresponds to the formation of a covalent bond by the transfer of a pair of electrons.
Lewis Base
A species which can donate a pair of electrons. It is a nucleophilic reagent.
Examples: NH3, H2O.
Gilbert Newton Lewis
An American physical chemist (1875-1946). He defined the covalent bond as resulting from sharing a pair of electrons. He also developed a theory of acids and bases.