electron-deficient bond

https://doi.org/10.1351/goldbook.E01985
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 a 'two-electron three-centre bonds'.
Source:
PAC, 1994, 66, 1077. (Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)) on page 1110 [Terms] [Paper]