When two s type orbitals overlap, they form a sigma (σ) molecular orbital. A sigma molecular orbital is formed when the orbitals overlap head-on, meaning that the lobes of the orbitals are pointing directly at each other. The resulting molecular orbital is symmetrical about the internuclear axis, meaning that it has the same electron density on both sides of the nucleus.

Sigma molecular orbitals are the most stable type of molecular orbitals, and they are typically the first to form when atoms bond. They are also the strongest type of molecular orbitals, and they are responsible for holding atoms together in a molecule.

In the case of two s type orbitals, the overlap is said to be "perfect," meaning that the orbitals overlap completely. This results in the formation of a very strong sigma molecular orbital. The sigma molecular orbital that is formed when two s type orbitals overlap is called the "s sigma" molecular orbital, or simply the "σs" molecular orbital.