The intergalactic drive was a theoretical piece of technology allowing easy travel over the distances between one galaxy and another.
The Federation did not appear to have intergalactic drive technology but Avon implied it was being worked on. The Andromedan aliens who attacked the Federation in the Intergalactic War presumably had the drive fitted to their invasion fleet. (B: Star One)
It is speculated that intergalactic drive may have some unavoidable limitation, which allow ships, making intergalactic travel, to arrive only in certain, predictable points. Only this assumption could make sence out of such defense measures as Antimatter minefield.