In the Star Trek fictional universe, shields refer to a 23rd and 24th century technology that provides starships, space stations, and entire planets with limited protection against damage.
The term 'shields' first appears in the Star Trek episode "Balance of Terror", in which they were deployed, albeit to little effect, by a Federation outpost under attack from a Romulan Warbird.
Characters discuss its existence and manipulation, while only superficially describing its exact physics, which result in a field being projected around a ship or other body, deflecting or dispersing projectiles and energy weapons.
In The Making of Star Trek, authors Gene Roddenberry and Stephen Whitfield write that shields are force fields similar in nature to the navigational deflector, while Mr. Scott's Guide to the Enterprise further states that shields on post-refit Constitution-class vessels - of which the USS Enterprise of Star Trek: The Original Series is an example - were generated by the subatomic scan and replication of an alloy known as diburnium-osmium, and then projected as a force field beyond a ship's hull along the shield grid.
The USS Defiant, introduced in the third season of Deep Space Nine, was said to use ablative armor in addition to shields; this technology also appeared in the alternate time line of the early 25th century shown in the Star Trek: Voyager series finale "Endgame", where it withstood repeated assaults by Borg weaponry and tractor beams.