Share. By Anne Marie Helmenstine, Ph.D. Definition: A pi 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.hare. By Anne Marie Helmenstine, Ph.D. Definition: A pi 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.