A projected barrier of energy used throughout Star Trek to hold prisoners, seal hull breaches, contain hazards and protect people and equipment from harm.
Force fields are among the most common tools in Star Trek. Emitters throw up an invisible wall that flashes when touched. The same principle lies behind deflector shields, structural integrity fields and the containment fields that hold antimatter in a warp core.
In the 22nd century the technology is new to humans: in the Enterprise episode Vox Sola, Malcolm Reed builds a prototype field to hold back an alien organism. By Kirk's time the brig of the Enterprise is closed by a field instead of bars.
On 24th-century ships the computer raises emergency force fields within moments of a hull breach, as seen in The Next Generation, Deep Space Nine and Voyager. Sickbay uses them for quarantine and security teams use them to seal corridors against boarders such as the Borg, who quickly adapt and walk through.