The Colonial Fleet unit that battlestar Galactica was assigned to at the time of the Cylon attack. It is often abbreviated (including on the Galactica's crest) as "BSG 75."
Battlestar Group is seen by RDM as analagous to a modern-day Carrier Battle Group, comprising at least one battlestar, plus a number of support vessels. Given the number of available battlestars at the outbreak of hostilities with the Cylon attack, it is possible that a battlestar group may have comprised a number (3-4) battlestars operating as a single unit, supported by additional Colonial vessels.