The ship-mounted energy weapon installed on Enterprise NX-01 in 2151, the forerunner of the phaser bank and far more powerful than the plasma cannons it replaced.
Phase cannons are the main energy weapons of Enterprise NX-01. The ship leaves Earth in a hurry in Broken Bow without them, relying on spatial torpedoes that soon prove inadequate.
In Silent Enemy, late in 2151, an unknown alien ship attacks repeatedly. Rather than return home, Trip Tucker and Malcolm Reed's teams build and fit the cannons themselves. A test shot overloads and blasts the top off a mountain on a moon, and the same overload is then used on purpose to drive the aliens away.
The cannons are rated at 500 gigajoules and extend from ports in the hull. They serve through the rest of the series alongside photonic torpedoes, and hand-held phase pistols work on the same principle.