An axis about which a set of ligands is held so that it results in a spatial arrangement which is not superposable on its mirror image. For example with an allene $\ce{abC=C=Ccd}$ the chiral axis is defined by the $\ce{C=C=C}$ bonds; and with an ortho-substituted biphenyl the atoms $\ce{C-1}$, $\ce{C-1'}$, $\ce{C-4}$ and $\ce{C-4'}$ lie on the chiral axis.