  We have compiled contemporaneous broadband observations of the black hole
candidate X-ray binary GX 339-4 when in the low/hard X-ray state in 1981 and
1997. The data clearly reveal the presence of two spectral components, with
thermal and non-thermal spectra, overlapping in the optical -- near-infrared
bands. The non-thermal component lies on an extrapolation of the radio spectrum
of the source, and we interpret it as optically thin synchrotron emission from
the powerful, compact jet in the system. Detection of this break from
self-absorbed to optically thin synchrotron emission from the jet allows us to
place a firm lower limit on the ratio of jet (synchrotron) to X-ray
luminosities of $\geq 5$%. We further note that extrapolation of the optically
thin synchrotron component from the near-infrared to higher frequencies
coincides with the observed X-ray spectrum, supporting models in which the
X-rays could originate via optically thin synchrotron emission from the jet
(possibly instead of Comptonisation).
