The Repulsor Field is a weapon introduced in Homeworld: Cataclysm.
The repulsor field emits a large amount of energy in a spherical movement that grows larger and repels any ship (except own ships, allies are affected by this) and slightly damages it. It was first shown in the final mission where the Nomad Moon was using it to protect itself from enemy ships. Smaller ships are extremely vulnerable to a repulsor field and tend to explode when hit.
As the name suggests, the repulsor field is used to repel enemy ships. Once fired it releases energy that repels and damages ships caught in it. The Dreadnought was the second ship to use repulsor technology: however, this reduced its Ion Cannon coverage. It seems that the field's radius depends on the amount of released energy and the size of the ship, as the Nomad Moon's repulsor field was enormous, but the Dreadnaught's field wasn't that large. Smaller ships can be sent flying, but larger ships can be pushed slightly. Also, the repulsor field can be used to deflect the particle fired by a Siege Cannon.