An atom or group that forms a bond to what is considered to be the main part of the @S06082@ during a reaction. For example: the attacking @N04249@ in a @M03989@ nucleophilic @S06078@.
Source: PAC, 1994, 66, 1077. (Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)) on page 1113 [Terms] [Paper]