  We define and characterize a photometric bandpass (called "Y") that is
centered at 1.035 um, in between the traditionally classified ``optical'' and
``infrared'' spectral regimes. We present Y magnitudes and Y-H and Y-K colors
for a sample consisting mostly of photometric and spectral standards, spanning
the spectral type range sdO to T5V. Deep molecular absorption features in the
near-infrared spectra of extremely cool objects are such that the Y-H and Y-K
colors grow rapidly with advancing spectral type especially from late M through
mid L, substantially more rapidly than J-H or H-K which span a smaller total
dynamic range. Consistent with other near-infrared colors, however, Y-H and Y-K
colors turn blueward in the L6-L8 temperature range with later T-type objects
having colors similar to those of warmer M and L stars. Use of the Y-band
filter is nonetheless promising for easy identification of low-mass stars and
brown dwarfs, especially at young ages. The slope of the interstellar reddening
vector within this filter is A_Y = 0.38 x A_V. Reddening moves stars nearly
along the YHK dwarf color sequence making it more difficult to distinguish
unambiguously very low mass candidate brown dwarf objects from higher mass
stars seen, e.g. through the galactic plane or towards star-forming regions.
Other diagrams involving the Y-band may be somewhat more discriminating.
