A Lewis base is any substance that can donate a pair of non-bonding electrons.
In 1923 G. N. Lewis suggested another way of looking at the reaction between H+ and OH- ions. In the Lewis theory of acid-base reactions, bases donate pairs of electrons and acids accept pairs of electrons.
A Lewis acid is therefore any substance, such as the H+ ion, that can accept a pair of non-bonding electrons. In other words, a Lewis acid is an electron-pair acceptor. A Lewis base is any substance, such as the OH- ion, that can donate a pair of nonbonding electrons. A Lewis base is therefore an electron-pair donor.
One advantage of the Lewis theory is the way it complements the model of oxidation-reduction reactions. Oxidation-reduction reactions involve a transfer of electrons from one atom to another, with a net change in the oxidation number of one or more atoms.
It should have lone pair of electrons
It should have lone pair of electrons that it can donate