A @CT01038@ or @M03986@ having an available pair of electrons capable of forming a @C01384@ with a @H02904@ (@P04906@) (see @B00745@) or with the vacant orbital of some other species (see @L03511@).
See also:
hard base
PAC, 1990, 62, 2167. (Glossary of atmospheric chemistry terms (Recommendations 1990)) on page 2176 [Terms] [Paper]
PAC, 1994, 66, 1077. (Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)) on page 1088 [Terms] [Paper]