Lewis base

https://doi.org/10.1351/goldbook.L03511
A @M03986@ (and the corresponding @CT01038@) able to provide a pair of electrons and thus capable of @C01329@ to a @L03508@, thereby producing a @L03510@.
Source:
PAC, 1994, 66, 1077. (Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)) on page 1135 [Terms] [Paper]