NCD2539 is an object in space near a Cylon supply route which is known to the Colonials.
Galactica intentionally visits this area to lure Cylon forces, including a Resurrection Ship, into to it. The Colonials intend to kill their humanoid Cylon prisoners, whose consciousnesses would download and infect the Cylon populace with a human virus with a bioelectric pathogen that would effectively annihilate the Cylon race. The objectives of the Battle of NCD2539 are thwarted by internal sabotage (TRS: "A Measure of Salvation").
The complete designation of the battle's location is "The Region of NCD2539 - Above/North."
- "NCD2539" is likely a Colonial astronomical catalog's designation for a specific celestial object; for example a star system. Such designations are common, because there are far too many stars to name all of them. "Above/North" refers to a direction perpendicular to the galaxy's disk in relation to its spin direction.