  Near the central engine, many astrophysical jets are expected to rotate about
their axis. Further out they are expected to go through the processes of
reconfinement and recollimation. In both these cases, the flow streams along a
concave surface and hence, it is subject to the centrifugal force. It is well
known that such flows may experience the Centrifugal Instability (CFI), to
which there are many laboratory examples. The recent computer simulations of
relativistic jets from Active Galactic Nuclei undergoing the process of
reconfinement show that in such jets CFI may dominate over the Kelvin-Helmholtz
instability associated with velocity shear (Gourgouliatos and Komissarov 2018).
In this letter, we generalise the Rayleigh criterion for CFI in rotating fluids
to relativistic flows using a heuristic analysis. We also present the results
of computer simulations which support our analytic criterion for the case of an
interface separating two uniformly-rotating cylindrical flows. We discuss the
difference between CFI and the Rayleigh-Taylor instability in flows with curved
streamlines.
