Stealth technology

Stealth technology was a description used of several types of technology used to remain undetected from sensors.

History
In 2151, the Klingon courier Klaang used stealth technology to hide his scout ship from Human and Vulcan sensors. 

Later that year, the encountered a Xyrillian starship that utilized stealth technology. T'Pol was able to analyze their stealth telemetry and locate the ship. 

In an alternate timeline stealth technology was used by the Federation on Starfleet spacecraft in the 25th century.

In 2404, Admiral Kathryn Janeway, from the alternate timeline, brought future stealth technology with her on her SC-4 shuttlecraft when she time traveled back to the year 2378. She intended the technology to be used on the USS Voyager, so that the ship could enter one of the Borg's transwarp hubs without incident. The stealth technology was however found to be incompatible with Voyager's systems by Seven of Nine.