Wiktionary
n. (context star English) A star that is dimmer than a main sequence star of same temperature. They are located below the main sequence in the Hertzsprung-Russell diagram.
Wikipedia
A subdwarf, sometimes denoted by "sd", is a star with luminosity class VI under the Yerkes spectral classification system. They are defined as stars with luminosity 1.5 to 2 magnitudes lower than that of main-sequence stars of the same spectral type. On an Hertzsprung–Russell diagram subdwarfs appear to lie below the main sequence.
The term "subdwarf" was coined by Gerard Kuiper in 1939, to refer to a series of stars with anomalous spectra that were previously labeled as "intermediate white dwarfs".
Usage examples of "subdwarf".
Each subdwarf tried to find a new home, but the more civilized dwarves resident around the little sea were not eager hosts.
One subdwarf took over the island marked Crete which, a verbal footnote explained, had recently suffered a terrible calamity that stunned its own civilized dwarf and rendered him helpless by also sinking his ship.
In a few days that subdwarf took the name of Philistine and raided the fertile riverland of Egypt, but he was driven off by the resident dwarf.