A spectroscopic binary is a binary star system which appears to be a single point of light, but for which movement of one of the pair is detectable by noting Doppler shifts in the spectrum indicating movement to and from the observer.
It is one of a number binary-star classes based upon how the star is determined to be binary. Others:
A double-line spectroscopic binary is one where a shifting spectral line can be seen for each star. A single-line spectroscopic binary (or single-lined spectroscopic binary) is one where shifting lines are seen only for one of the two stars, i.e., one star is so bright that the other cannot be seen. The observation will be very similar to a single star with an extra-solar planet discovered by the radial velocity method, and some effort is now necessary to distinguish the two.