A quantum matrix is the quantum structure of matter.
The quantum matrix of the planet Meridian fluctuated, causing it to shift into our dimension for a brief period every sixty years. In 2371, when Jadzia Dax opted to remain on Meridian, the transporter buffer was used to bring her quantum matrix in sync with the planet's during the its next shift. However, the planet's matrix began to destabilize due to her presence, forcing Chief O'Brien to beam her back to the USS Defiant. (DS9: "Meridian")