Term used mainly in the German literature for starting material (reactant). It should be avoided in English, because there it means 'something that comes out' and not 'something that goes in'. The German use of the term is in fact also incorrect.
Source:
PAC, 1994, 66, 1077. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)' on page 1108 (https://doi.org/10.1351/pac199466051077)