The relative ability of a species to donate a lone pair and form a new bond is called nucleophilicity.

In contrast to basicity, which is measured by the position of acid-base equilibrium, nucleophilicity is generally measured by reaction rate. 

Many, but not all, of the same factors that influence basicity also influence nucleophilicity. For example: charge, the identity of the atom, resonance stabilization, the presence of electron-donating or withdrawing groups, and the s-character of the atom bearing the lone pair.

However, since the atom being attacked is usually carbon, nucleophilicity is more sensitive to steric effects than is basicity.