  We report a detection in the interstellar medium of an infrared transition
within the electronic ground state of the deuterated hydrogen molecule, HD.
Through a deep integration with the Short-Wavelength-Spectrometer on ISO, the
pure rotational v=0-0 R(5) line at 19.43um was detected toward the Orion
(OMC-1) outflow at its brightest H_2 emission region, Peak 1. The ~20"
beam-averaged observed flux of the line is (1.84 +- 0.4) 10^-5 erg cm^-2 s^-1
sr^-1. Upper flux limits were derived for sixteen other rotational and
ro-vibrational HD lines in the wavelength range 2.5 to 38 um. We utilize the
rich spectrum of H_2 lines observed at the same position to correct for
extinction, and to derive a total warm HD column density under the assumption
that similar excitation conditions apply to H_2 and HD. Accounting for non-LTE
HD level populations in a partially dissociated gas, our best estimate for the
total warm HD column density is N(HD)=(2.0+-0.75)10^16 cm^-2. The warm
molecular hydrogen column density is (2.21+-0.24)10^21 cm^-2, so that the
relative abundance is [HD]/[H_2]=(9.0+-3.5)10^-6. Accounting for HD depletion
relative to H_2 in partially dissociative shocks we derive a deuterium
abundance in the warm shocked gas, [D]/[H]= (7.6+-2.9)10^-6. Our implied
deuterium abundance is low compared to previous determinations in the local
interstellar medium, but it is consistent with two other recent observations
toward Orion, suggesting that deuterium may be significantly depleted there.
