Kinematic redshift ($1+z_{\mathrm{kin}}=\sqrt{(1+v/c)/(1-v/c)}$)

A kinematic redshift is the relativistic Doppler effect from relative motion of emitter and receiver. With positive radial recession speed $v$, its small-velocity limit is $z_{\mathrm{kin}}=v/c+O(v^2/c^2)$. A peculiar velocity gives such a factor in addition to cosmological redshift.