The Topin system was a star system near the Demilitarized Zone between the Cardassian Union and the United Federation of Planets. The system contained an unstable protostar which generated an enormous amount of magnetascopic interference rendering most sensors and communications unreliable.
In 2370 the USS Enterprise-D responded to a distress call from a Federation science vessel originating in the Topin system. The distress call was in fact a ploy by Ro Laren to lure the Enterprise to the system for the purpose of stealing medical supplies which she transferred to the Maquis. (TNG: "Preemptive Strike")