A Nova Cannon is a high powered weapon almost exclusively encountered on Imperial vessels.
The principle of such a weapon is very similar to that of a rail-gun, in that it requires no propellant or fuel to move the projectile. Instead, the ammunition is accelerated using powerful magnetic fields to speeds almost equal to the speed of light.
The recoil generated by such a high energy release practically prohibits vessels from mounting nova cannons on turrets, or on smaller ships. Thus, they are nearly always fired in a fixed forward arc, where the powerful ships engines can compensate for the reverse motion of the projectile.
The ammunition fired by the nova cannon is a specially prepared imploding charge, timed to detonate after it achieves a set distance from the vessel that fired it. The distance is calculated by the ship's weapons officer. Though the high speed of the projectile prevents detonation at short ranges, as it will have already obtained such a high distance in a very short space of time.
There's also an unrelated plasma-based Nova Cannon which is mounted on the Ordinatus Armageddon.