The axisymmetric synthetic jet produced by an oscillating piston has been investigated experimentally. The study was performed in water using digital particle image velocimetry for the instantaneous velocity planar measurements. The near field, up to x/d0 ≈ 6, was investigated for three different Reynolds numbers. The self-similarity of the flow field is already fully reached for the mean velocity profiles at x/d0 ≈ 4 while longer distances are needed for the turbulent intensity profiles. The flow structure investigation shows that a single vortex ring or a train of vortex rings characterizes the near field during the blowing phase, depending on the jet flow parameters. The details of the flow field have also been described in terms of phase-averaged flow analysis and of triple decomposition of the instantaneous velocity field. The near flow fields investigated do not seem to be influenced by the Reynolds number.