Flux density is a type of flux measurement used to determine density. Two types of flux densities included gravimetric flux density and magnetic flux density. Magnetic flux density could be determined by a magnetic flux density analysis.
A high magnetic flux density reading could be a result of an electromagnetic storm. In 2367, it was determined that the anionic energy remnants of the Ux-Mal criminals found on on the barren moon of Mab-Bu VI were vulnerable to the magnetic flux density found in the storms that plagued the moon. (TNG: "Power Play")
In 2369, Chief Miles O'Brien told the senior staff of Deep Space 9 that the Federation had developed a far better way to control flux density with regard to pulse wave torpedoes in the last two hundred years. (DS9: "If Wishes Were Horses")
A high gravimetric flux density reading could be a result of a quantum singularity. In 2371, the USS Voyager discovered a type 4 quantum singularity with a gravimetric flux density of 2,000%. (VOY: "Parallax")