A single bond between adjacent atoms that is formed by less than two electrons, as in
B2H6:
E01985.png
The
B–H–B bonds are called 'two-electron three-centre bonds'.
Source:
PAC, 1994, 66, 1077. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)' on page 1110 (https://doi.org/10.1351/pac199466051077)