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 (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.