A Lewis base with a donor centre (e.g. an oxygen atom) of low polarizability the converse applies to soft bases.
See also: hard acid
Source:
PAC, 1994, 66, 1077. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)' on page 1120 (https://doi.org/10.1351/pac199466051077)