In Star Trek: Deep Space Nine, the Defiant uses a cloaking device to become nearly invisible to ordinary sensors.
A cloaking device conceals a vessel by manipulating electromagnetic and other detectable emissions. The Defiant receives a Romulan-built cloaking device under an agreement that permits its use in the Gamma Quadrant. This arrangement is unusual because the Federation generally restricts cloaking technology under the Treaty of Algeron.
The device gives the Defiant a major tactical advantage during reconnaissance and combat. It does not make the ship absolutely undetectable: specialized sensors, unusual phenomena, and certain technologies can reveal or interfere with a cloaked vessel.
Cloaking is different from a deflector shield. Shields protect a ship from incoming hazards, while cloaking primarily hides its presence. It is also distinct from a stealth field, a broader term sometimes used for concealment systems in other science-fiction settings.