3-center-2-electron bond

This is an electron deficient bond where three atoms share two electrons. The combination of three atomic orbitals form one bonding, one non-bonding, and one anti-bonding molecular orbital. Since the two electrons go into the bonding orbital this constitutes a chemical bond between the three atoms. These types of bonds are found in the case of a bridging hydride or methyl. See trimethylaluminium and diborane.