Delta band emissions were energy emissions given off by Starfleet tricorders.

During a visit to a Romulan Warbird that was trapped in time, Lieutenant Commander Data scan emissions caused the temporal aperture that the ship was trapped in to activate, and time resumed.

Later, Captain Jean-Luc Picard had Data intentionally activate the tricorder to rewind time far enough to prevent the warp core breach that was frozen in progress. Data figured that he "could attempt to remodulate the tricorder's delta band emissions," so as to "better control the temporal aperture." (TNG: "Timescape")

Delta band emissions were not originally mentioned in the episode's final draft of the script, which instead stated, he "can attempt to modify the tricorder's scan frequency," as it "might allow us to manipulate the temporal aperture's effects."
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