In chemistry, delta bonds (δ bonds) are chemical bonds of the covalent type, where four lobes of one involved electron orbital overlap four lobes of the other involved electron orbital. Of the orbital's node planes, two (and no more) go through both atoms.
In chemistry, in sufficiently-large atoms, occupied d-orbitals are low enough in energy to participate in bonding. Delta bonds are usually observed in organometallic species. Some ruthenium and molybdenum compounds contain a quadruple bond, which can only be explained by invoking the delta bond.
It is possible to excite electrons in acetylene from lower-energy nonbonding orbitals to form a delta bond between the two carbon triple bonds. This is because the orbital symmetry of the pi antibonding orbital is the same as that of the delta bond.
Presumably, higher-order bonds (phi bonds and gamma bonds, compare to f and g orbitals) are possible, with even more overlapping lobes of their component atomic orbitals, but these have not been observed. Occasionally, one encounters these higher-order bonds in theoretical chemistry.