In General Chemistry, pi bonds (π bonds) are covalent electron bonds where two lobes of one involved electron orbital overlap two lobes of the other involved electron orbital. What this means is, when two atoms are sufficiently close and when they each have one electron that is not being shared, the two electron orbits can essentially "overlap" causing a new bond between the atoms. Any atom capable of forming more than one electron can form pi bonds, so it is relatively common to find pi bonds in molecules containing two or more nitrogen or carbon atoms, but one would never find (in nature) pi bonding between hydrogens, halogens or other atoms which prefer to share only one electron.
The Greek letter π in the name refers to p orbitals, since the orbital symmetry of the pi bond is the same as that of the p orbital when seen down the bond axis. P orbitals most often engage in this sort of bonding, however, d orbitals can also engage in the formation of pi bonds.
Pi bonds are usually weaker than sigma bonds (σ bonds) because pi bond orbitals extend further from the positive charge of the atomic nucleus, which therefore requires additional energy. From the perspective of quantum mechanics, this bond weakness is explained by significantly less overlap between what were previously p orbitals due to the parallel orientation of the orbitals. The pi bond by itself is weaker than a sigma bond, but pi bonds are only found in combination with sigma bonds, so the combination of the two bonds is 'stronger (and shorter in length) than either bond would be by itself.
Two or More p-Orbitals Can Form a π-BondEdit
Atoms with double bonds or triple bonds as found in nature always have one sigma-type bond and the rest pi-type bonds. Pi bonds result from parallel orbital overlap: the two combined orbitals meet lengthwise and create more diffuse bonds than the sigma bonds. No commonly-occurring natural substance has more than three bonds to another atom, and if there are three bonds it is safe to assume that the three bonds consist of one sigma bond and two pi bonds.
It is important to note that atoms will not generally form more bonds than predicted by their place in the Periodic Table of Elements. Electrons in pi bonds are also often referred to as "pi electrons".