  The entrainment of ambient fluid into a turbulent shear flow has been a topic
of wide interest for several decades. To estimate the entrainment of ambient
fluid into turbulent jet, it is essential to define the boundary of the jet.
The question arises as to what should be the appropriate criterion to determine
the edge of the turbulent jet. From the present DNS simulations, we observe
that there is a need to define two boundaries for the turbulent jet, namely,
the Rotational/Irrotational (R/IR) interface termed here as "Outer boundary"
and the Turbulent/Non-Turbulent (T/NT) interface as "Inner boundary". The main
aim of the present paper is to identify the need for defining the two
boundaries for the jet. We also present some interesting observations of the
jet flow, essentially on self similarity and self preservation.
