A plasma wake was an ion trail left by the warp engines of starships traveling through nebular gases.
The Jem'Hadar were able to detect the cloaked USS Defiant while it traveled through the Badlands in 2373 by homing in on its plasma wake. Jadzia Dax compared it to a Ligorian mastodon moving through the forest – "you may not be able to see it, but it definitely makes the leaves rustle." (DS9: "Blaze of Glory")