A force field was an energy barrier with many applications and varying degrees of strength. Although force fields had been in use for many years, Starfleet did not begin research on such a device until 2147, then referred to by Malcolm Reed as a "stable EM barrier." By the 24th century, Starfleet force fields were commonplace and were rated by intensity, ranging in strength from level 1-10. A level 10 force field was the strongest, and would be used, for example, during a scientific experiment of which the outcome was unknown, or known to be explosive in nature. Applications range from creating holograms, to sealing a hull breach, to personal force fields designed to keep potential assailants at bay.
The effects of a force field on its surroundings varied greatly. Contact with a force field could cause anything from a slight tingle to death. Most force fields were non-lethal, although some civilizations, such as the Dominion, preferred the lethal variety. If a force field was active, an object or transporter beam generally could not pass through it.