Energy shielding

Energy shielding is any protective technology that utilizes energy field-based means (such as electromagnetism or gravitics) to deflect incoming damage. Energy shielding can be either passive (always on) or active (activated only upon imminent impact), and may be designed to deflect either projectile- or energy-based strikes, or both.

UNSC
Most UNSC shield systems are active rather than passive, taking advantage of the UNSC's more lenient approach to AI and automation in comparison to the Covenant.

Proto-shields
Proto-shields are a category of electromagnetic impeller systems unrelated to Covenant shield technologies. Some of these have existed for quite some time even before first contact, though the military applications of these early systems were limited. Even prior to the Human-Covenant War, some UNSC ships and stations used localized electromagnetic fields to protect them from incoming radiation. Such systems were far too weak against Covenant plasma weaponry, however. Upscaled versions of the technology were used to shield several colonies without native magnetospheres, such as Mars and the Jovian Moons, from radiation.

Early "plasma windows" used to create pressure barriers in some fringe applications are a related technology.

Passive/semi-active conformal shielding
As soon as the UNSC came into contact with the Covenant, various projects sought to both reverse-engineer Covenant energy shielding as well as develop viable alternatives. The most successful of these was Project AEGIS, which eventually resulted in functional energy shielding by the late 2540s. This came with several limitations, however. The shields were most useful on a small scale, such as on a MJOLNIR battlesuit or small fighter craft. To be viable for sustained operation, they also require an onboard reactor. The larger the object being shielded, the more prohibitive the power consumption of the system.

Active dispersal fields
Developed during the mid-Covenant War based on old, long-mothballed designs, active dispersal fields use a targeted strong electrogmagnetic field to reduce the energy of incoming directed energy weapon attacks; against projectile-based attacks, they are useless. Unlike Covenant-style shields, the dispersal fields are not always on, which makes them more energy-efficient, but also requires a sophisticated threat-detection system to function. This makes them ideal for spacecraft, particularly large warships. Since the mid-war, proto-shield development was a gradient with the later years of the war already seeing fairly successful solutions though the technology would only mature and gain widespread adoption in the post-war period. When used in vehicle armor, they are usually known as shock plates, or jolt plates. By the 2550s, many UNSC vehicles incorporated such armor.

Strike shields are essentially dispersal fields that use a battery, rather than a fusion reactor, to function. They fall after a few strikes and require a battery recharge or replacement to replenish. However, they are far cheaper than shielding equipped with an onboard reactor, and increasingly widespread on various infantry armor systems over the 2560s.
 * Strike shielding