A sigma bond (σ bond) is a type of chemical bond. It is the strongest one of the covalent type. In a sigma bond, the atomic orbitals from two atoms which are nearby go one on top of each other. This is different from a pi bond where they touch sideways. This is why a sigma bond is stronger. Sigma bonds can involve s, p, d or f orbitals. They must however be of the right symmetry to be able to form the bond.