  We explore the laminar to turbulence transition of round jets at low Reynolds
number (Re < 1000) using a novel experimental setup and linear stability theory
(LST). The setup has a large domain and a low disturbance environment which
increases the critical Re to approximately 500, permitting the appearance of a
hitherto unknown puff-like instability (PFI). The instability is identified in
the self-similar region of the jet through clean flow visualizations (FV) and
further corroborated by particle image velocimetry (PIV) measurements. For 400<
Re <700, the flow exhibits PFI embedded in a puff-train encapsulated by a
laminar flow analogous to 'puffs' in pipe-flow transition; the latter being
symbolic of the finite-amplitude disturbance transition scenario. The
observation that PFI convects close to the average local velocity with an
inflectional velocity profile further strengthens the analogy. LST predicts
that PFI is effectively a superposition of the helical mode (HM) pair with
azimuthal wavenumbers n=+-1. Hence, PFI can also appear in the
infinitesimal-amplitude supercritical route to transition of linearly unstable
flows.
