Pi Bond Definition in Chemistry

Pi bonds are formed between two adjacent atom's p-orbitals.
Two adjacent atoms with unbonded p-orbital electrons can merge together to form an electron pair. This forms a pi bond. Todd Helmenstine

A pi bond (π bond) is a covalent bond formed between two neighboring atom's unbonded p-orbitals.

An unbound p-orbital electron in one atom forms an electron pair with a neighboring atom's unbound, parallel p-orbital electron. This electron pair forms the pi bond.

Double and triple bonds between atoms are usually made up of a single sigma bond and one or two pi bonds. Pi bonds are generally denoted by the Greek letter π, in reference to the p orbital.

The symmetry of a pi bond is the same as that of the p orbital as viewed down the bond axis. Note d orbitals also form pi bonds. This behavior is the basis of metal-metal multiple bonding.