Sid Meier's Alpha Centauri/Shielding

A unit's defense determines how well it fights when it is attacked. Defense strength affects a unit's mineral cost. Some defenses affect a unit's cost by more than just their defensive strength.

No Armor

 * Strength: 1

Psi Defense

 * Prerequisite: Eudaimonia
 * Strength: Psi
 * Cost: 6

Units equipped with a Psi Defense always engage in Psi combat when attacked, ignoring weapon and armor strengths. This is useful for defending against opponents whose weapon technology exceeds your defensive technology. Unfortunately, this armor is pitiful against conventional artillery so you will need to ensure that any such defenders (including native life) are within 1 square of good artillery, in this situation probably equipped with Psi Attack.

Synthmetal Armor

 * Prerequisite: Industrial Base
 * Strength: 2

Plasma Steel Armor

 * Prerequisite: High Energy Chemistry
 * Strength: 3

Pulse 3 Armor
''Same as Plasma Steel Armor, but more expensive and with +25% bonus vs. mobile attackers. Alien Crossfire only.''
 * Prerequisite: Adaptive Doctrine
 * Strength: 3
 * Cost: 5

Generally a bad choice of armor, since it costs just as much as Photon Wall, yet is less effective than Silksteel Armor even against mobile attackers. Still, it comes earlier in the tech tree than Silksteel, and might make the difference against an Impact Rover. Provides no benefit to ships except perhaps when defending in a base against a mobile land attacker, since ships are never mobile attackers. The AI prefers Pulse-3 Armor, which needlessly pushes up its unit costs. Disable it or decrease its cost to help out the AI.

Resonance 3 Armor
''Same as Plasma Steel Armor, but more expensive and with +25% bonus vs. Psi attacks. Alien Crossfire only.''
 * Prerequisite: Field Modulation
 * Strength: 3
 * Cost: 5

Mainly useful for combining with Hypnotic Trance on a foil for a great Psi defender. Useful for secondary base defenders, transports, formers and supply crawlers. Other than that, see above.

Silksteel Armor

 * Prerequisite: Silksteel Alloys
 * Strength: 4

Photon Wall

 * Prerequisite: Photon/Wave Mechanics
 * Strength: 5
 * Cost: 5

Probability Sheath

 * Prerequisite: Probability Mechanics
 * Strength: 6

Neutronium Armor

 * Prerequisite: Matter Compression
 * Strength: 8

Pulse 8 Armor
''Same as Neutronium Armor, but more expensive and with +25% bonus vs. mobile attackers. Alien Crossfire only.''
 * Prerequisite: Super Tensile Solids
 * Strength: 8
 * Cost: 11

Resonance 8 Armor
''Same as Neutronium Armor, but more expensive and with +25% bonus vs. Psi attacks. Alien Crossfire only.''
 * Prerequisite: Sentient Resonance
 * Strength: 8
 * Cost: 11

Antimatter Plate

 * Prerequisite: Matter Editation
 * Strength: 10

Stasis Generator

 * Prerequisite: Temporal Mechanics
 * Strength: 12