The standard ship-mounted and hand-held energy weapon of the Dominion, whose phased polaron beams passed straight through Federation shields when the Jem'Hadar were first encountered in 2370.
Polaron beams are the principal weapons of Jem'Hadar warships and rifles. At first contact they held a decisive advantage: the beams ignored Starfleet deflector shields entirely.
This is shown in the Deep Space Nine episode The Jem'Hadar, in which the Galaxy-class USS Odyssey is battered through its shields and then destroyed by a suicide run. The USS Defiant suffers the same problem on its first mission in The Search.
By the time open war breaks out in Call to Arms, Starfleet and its allies have modified their shields to resist polaron fire, and battles with the Dominion are fought on more even terms.