Delta Source was the codename of an Imperial intelligence source during the early years of the New Republic. It proved essential to providing inside information to Grand Admiral Thrawn, which he used to further his campaign against the fledgling republic.
A remnant of Emperor Palpatine's reign, Delta Source was one of many hidden systems scattered across Coruscant that the Emperor used to monitor his courtiers and officers. The system was composed of twin rows of ch'hala trees in the Grand Corridor of Imperial Palace. A communications tube buried in the taproot of one of the trees converted the audio-activated ripples of the tree into sound. The converted signals were then transmitted over maintenance chatter.
During his campaign, Thrawn leaked information about the existence of Delta Source, causing the New Republic leadership to believe that there was a traitor amongst their ranks. Later investigation led Winter and Zakarisz Ghent, tasked with uncovering the source, to believe it was a mechanical espionage system(with blame even briefly falling upon the shoulders of droids), until Leia Organa Solo discovered the ch'hala trees' purpose while watching a droid pruning its leaves.
Although the Rebel Alliance and New Republic did not discover Delta Source until Thrawn's leadership of the Empire, the spy system had actually been in place since before the outbreak of the Clone Wars.