Pi bond
In chemistry, pi bonds (π bonds) are bonds with a single nodal plane containing the line segment between the two atoms.
Pi bonds are named after the Greek letter "π", as in p orbitals, since the orbital symmetry of the pi bond is the same as that of the p orbital (when observed down the bond axis). P orbitals usually engage in this sort of bonding. However, d orbitals and even sigma bonds can engage in pi bonding.
Pi bonds are usually weaker than sigma bonds because their orbitals go further from the positive charge of the nucleus, which requires more energy. Quantum mechanically this bond weakness is explained by significantly less overlap between the previously p-orbitals due to their parallel orientation. Although the pi bond itself is weaker than a sigma bond, pi bonds are only found in multiple bonds in conjunction with sigma bonds and collectively they are stronger than either single bond. Atoms with double bonds or triple bonds have one sigma bond and the rest are usually pi bonds. Pi bonds result from parallel orbital overlap: the two combined orbitals meet lengthwise and create more diffuse bonds than the sigma bonds. Electrons in pi bonds are sometimes referred to as pi electrons.