The subspace field matrix was an aspect of a starship's warp field.
In 2374, while trying to recalibrate USS Voyager's warp field to resist a nebula's subnucleonic radiation, Seven of Nine recruited holographic versions of the crew to assist her. The holographic Neelix commented that the subspace field matrix of her modifications seemed right. (VOY: "One")
While conducting repairs on the Hysperian starship Monaveen in 2381, Ensign Sam Rutherford found that the subspace field matrix in the primary fusion reactor was offline, which should have been impossible as that would leave the plasma levels completely unregulated. As the Hysperians renamed everything to sound like magic, he tried calling it the "elf matrix", but was told to stop by Lieutenant Commander Andy Billups. (LD: "Where Pleasant Fountains Lie")