Gridfire is a weapon from the Culture novels written by Iain M. Banks. To understand what Gridfire is, it is important to understand what the Grid is in the Cultureverse. In this scifi universe, normal space when viewed in a Fifth dimension, is a membrane in a further space. It is stretched out between two sheets that look like an ocean throbbing with energy. These two sheets are the Energy Grid that underlie the universe. Culture ships and habitats use the Grid as a source of energy, siphoning power from it and using that power to move at c+ speeds.

They propel themselves by pushing against the Grid and bouncing back and forth between ultraspace and infraspace, those hyperspaces between the universe and the grid, forming a sort of 5 dimensional sine wave. This allows them to reach cruising speeds of 120 kilolights. There is also the case of the ROU (Rapid Offensive Unit) Killing Time, which at one point brought itself up to a speed greater than this by four orders of magnitude.

Though it can be activated with very high precision using a special weapons module, Gridfire can be created crudely by a very simple means. That is, by simply breaking very, very hard against the Energy Grid, a wave of the Grid is pushed up, enough force will push the wave into the universal 'plane.'

The energies released by this are astronomical. Reaching energies in excess of 2.240E+32 Joules on hitting planet sized bodies, to the point of their destruction. As is pointed out by the Xenophobe in Use of Weapons, even a deMilitarized Picket Ship can destroy a planet.