Field density
Field density was a scientific concept that was somehow related to gravity phenomena. 

The field density between the universe and the mirror universe was important in one method to cross over. In 2267, an away team from the  crossed over into the mirror universe. Montgomery Scott used the  to cross back over, requiring having a large field density. ()

In 2269, field density was an interest of Starfleet Commander Spock. It had also been studied by the android Rayna Kapec (along with such other studies like sub-dimensional physics) who had learned the concepts from the immortal Human Flint. ()

The term field density was also used as a unit of measurement for phase coils. Geordi La Forge's warp drive modifications made to the  included the phase coils upgraded to 55 field densities. ()

For his metaphasic shielding report, Doctor Reyga obtained the mean field density of the metaphasic shield from sensor readings conducted by a number of measurement probes during experiments. ()

See also 
Magnetic field
Science
Measurements