In subspace physics, subspace harmonics described a series of oscillations.
According to the Engineering Systems Database, there was an association between subspace harmonics and the warp field generated by a Federation starship. These ships monitored harmonics with subspace harmonic monitors. (TNG: "Booby Trap", display graphic)
Subspace harmonics were among the components of a warp signature. (DS9: "Defiant")
A subspatial transkinetic analysis used an algorithm based on subspace harmonics. (VOY: "Counterpoint")