In Star Trek: The Original Series, the defensive technology that surrounds a starship with protective energy barriers is the deflector shield.
Deflector shields protect spacecraft from weapons fire, radiation, debris, and other hazards. In combat, a shield absorbs or redirects energy from phasers, disruptors, and torpedoes until its strength is reduced. Star Trek frequently represents shield status as a measurable percentage or remaining capacity.
The term is different from a navigational deflector. A navigational deflector is a forward-facing system that clears dangerous matter from a ship’s path during flight. A deflector shield, by contrast, is a defensive barrier surrounding the vessel or a particular section of it.
Later Star Trek series add related systems, including structural integrity fields and regenerative shields, but the basic shield concept was already established in the original series. Shield technology is one of the franchise’s standard explanations for how starships survive battles that would otherwise destroy their hulls.