A sigma bond is the strongest type of covalent bond, formed by direct overlap between atomic orbitals along the axis connecting two atomic nuclei.
Pi Bond (π): This is another type of covalent bond formed by sideways overlapping of atomic orbitals above and below the plane of atoms' nuclei.
This refers to combining different types of atomic orbitals into an equivalent number of hybridized orbitals which have similar energy levels but different shapes and orientations than original ones.
It's the average distance between nuclei of two bonded atoms in a molecule.