  We present time resolved spectral analysis of prompt emission from GRB
160625B, one of the brightest bursts ever detected by Fermi in its nine years
of operations. Standard empirical functions fail to provide an acceptable fit
to the GBM spectral data, which instead require the addition of a low-energy
break to the fitting function. We introduce a new fitting function, called
2SBPL, consisting of three smoothly connected power laws. Fitting this model to
the data, the goodness of the fits significantly improves and the spectral
parameters are well constrained. We also test a spectral model that combines
non-thermal and thermal (black body) components, but find that the 2SBPL model
is systematically favoured. The spectral evolution shows that the spectral
break is located around $E_{\rm break}\sim$ 100 keV, while the usual $\nu
F_{\nu}$ peak energy feature $E_{\rm peak}$ evolves in the 0.5-6 MeV energy
range. The slopes below and above $E_{\rm break}$ are consistent with the
values -0.67 and -1.5, respectively, expected from synchrotron emission
produced by a relativistic electron population with a low energy cut-off. If
$E_{\rm break}$ is interpreted as the synchrotron cooling frequency, the
implied magnetic field in the emitting region is $\sim$ 10 Gauss, i.e. orders
of magnitudes smaller than the value expected for a dissipation region located
at $\sim 10^{13-14}$ cm from the central engine. The low ratio between $E_{\rm
peak}$ and $E_{\rm break}$ implies that the radiative cooling is incomplete,
contrary to what is expected in strongly magnetized and compact emitting
regions.
