Also called a Bussard collector, this is a coloured and sometimes hemispherical device at forward end of warp nacelles on Federation Starships in Star Trek.  These devices consist of a series of high-energy magnetic coils charged with ionizing radiation and collect surrounding space gases (primarily hydrogen) whcih are then used to replenish the ship's deuterium fuel supplies. Only small amounts can be gathered, and it is designed as a supplementary fuel source, not a technique for complete, long term refuelling.

According to the Star Trek The Next Generation Technical Manual, the ramscoop assembly consists of three main elements: an ionizing beam emitter (IBE, which charges neutral particles in space), a magnetic field generator / collector (MFG/C, a set of coils that cast a magnetic "net" ahead of the ship and pull in the charged particles ), and the continuous cycle fractionator (CCF, which separates the incoming gases). 

The device is named after an actual 20th century physicist Robert Bussard, who suggested a ramjet concept that coud provide, potentially, a highly effective form of interstellar propulsion.  Called the Bussard or interstellar ramjet neatly avoids this problem by harvesting hydrogen for use as a propellant from the interstellar medium. The captured hydrogen is fed to a nuclear fusion reactor which supplies the energy for a high-speed exhaust. The hydrogen ahead of the spacecraft is first ionized using a powerful laser. The hydrogen ions – naked protons – can then be drawn in by a relatively small Bussard collector that generates a powerful magnetic field. Since the harvesting process is electromagnetic rather than mechanical, the scoop does not have to be solid (it can be a mesh) nor does it have to be unrealistically large, because the field can be arranged to extend far beyond the physical structure of the scoop.