Force Fields
Although force fields had been in use for many years, Starfleet did not begin research on such a device until around 2147, then referred to by Malcolm Reed as a "stable EM barrier." By about 2152, Starfleet hadn't found a method of controlling the particle density of the field.
By the 24th century, Federation Starfleet force fields were commonplace and were rated by intensity, ranging in strength from levels 1 through 10. A level 10 force field was the strongest and was used, for example, during a scientific experiment of which the outcome was unknown, or known to be explosive in nature. Applications ranged from creating holograms to sealing a hull breach to personal force fields designed to keep potential assailants at bay.
Courtesy of Memory Alpha,
Star Trek Encyclopedia (4th ed., vol. 1, p. 280),
(TNG: "Realm of Fear", "The Most Toys"; Star Trek Generations; Star Trek: Insurrection; DS9: "Playing God", "Starship Down", "Take Me Out to the Holosuite"; VOY: "State of Flux", "Course: Oblivion")
