New Series
According to information releaed by Zoic Studios, Pacifica is the name of one of the original 12 battlestars built by the Colonials during the Cylon War. There is currently no canonical data to support this, but as the Pacifica was a name used in the the original series, it is probable the name would be carried forward to the new series.
The Original Series
Though not specifically seen on screen, the Pacifica, Acropolis, and Triton are mentioned when the Cylons attack the fleet at Cimtar (Saga of a Star World), so it can be assumed that these three were the three unnamed battlestars seen with the Atlantia and Galactica at Cimtar.