A deflector field is the energy field created by the deflector array of a starship. A deflector field is typically initialized automatically, though in extreme circumstances, it has to be activated manually.
When the USS Defiant's computer systems were rendered inoperable by a cascade virus in 2373, the deflector field had to be manually initialized by the bridge crew when the ship undocked from Deep Space 9. (DS9: "For the Uniform")
The deflector field had to be manually activated on a severely damaged USS Voyager after a Krenim scientist altered the past in 2374. (VOY: "Year of Hell, Part II")