A warp trail, also known as an ion trail, is a wake in subspace which is left by a starship using warp drive. A warp trail can be detected by sensors, but their are ways of masking them, such as traveling through a nebula.
Shortly after entering an unnamed star system near the Klingon Neutral Zone, Lieutenant Gary Mitchell aboard the USS Constitution detected an ion trail in orbit of one of the system's three planets. After following the trail, the Constitution was met by a Klingon D7-class battle cruiser, the IKS Stormwind. (TOS - My Brother's Keeper novel: Constitution)