SIGMA BOND :

                In chemistry sigma bonds are strongest type of covalent chemical bond. They are formed by direct overlapping between two adjacent atoms in the outer most orbitals. i.e Sigma bonds are the strongest type of covalent bonds due to direct overlap of orbitals and the electrons. In these bonds are sometimes reffered to as sigma electrons.Sigma bonds are most clearly defined for diatomic molecules using the language and tools of symmetry groups. Common forms of sigma bonds are S+S, Pz+Pz, S+Pz, and dz2+dz2.

                     Z ==> Defined as the axis of  bond.
sigma symbol is denoted by the Greek letter .