A pi (π) bond is a covalent bond formed by side-on overlap of p orbitals above and below the line joining the nuclei. Because the electron density lies off the bond axis, a pi bond does not permit free rotation. Pi bonds occur in double and triple bonds, where they accompany a sigma bond.

π 键是由 p 轨道在连接两核直线的上下方作侧向重叠而形成的共价键。由于电子密度位于键轴之外,π 键不允许自由旋转。π 键出现在双键和叁键中,在那里它们伴随一个 σ 键。