An antiproton beam is a confined beam of antiprotons.
The Doomsday Machine used an intense antiproton beam as its primary weapon, which could be used to destroy whole planets. (TOS: "The Doomsday Machine")
The Jem'Hadar used an antiproton beam to detect the USS Defiant through its Romulan cloaking device. The Cardassians later used this technique to limited effect when the Defiant was hijacked by the Maquis in 2371. (DS9: "The Search, Part I", "Defiant")