TNT equivalence is a method of measuring, expressing and comparing the energy released during violent events (typically, explosions). The TNT equivalent of an event is expressed as a mass, and represents the amount of trinitrotoluene whose detonation would be required to produce the amount of energy in question.
The method was originally used to express the power of nuclear detonations - as a means of 'rating' or 'grading' atomic weapons or explosions by their energy release. It may have arisen as a result of the '100 ton test.' On May 7, 1945, before the Trinity test, a conventional explosion was set off some few hundred yards from the planned Trinity site. It involved 108 tons of Composition B high explosive (a mix of TNT and RDX). The effects of that blast were used as a baseline for Trinity and subsequent detonations.
TNT, when detonated, releases somewhere between 4100 and 4600 joules of energy per gram, depending on the circumstances and the manufacture of the explosive. Probably since a thermochemical kilocalorie is 4184 joules, the United States National Institute for Standards and Technology (NIST) sets the 'standard' release of energy from one gram of TNT as 4184 joules, and thus the TNT equivalence of one ton of TNT at 4.184x109 joules of energy. This was not always the case; the U.S. Nuclear Weaponeering bible, Samuel Glasstone's 1977 The Effects of Nuclear Weapons, defines a kiloton of TNT as 4.2x109 joules.
It has become common to describe the effects of nuclear detonations and any other large-scale sudden energy release (comet/meteor impacts, for example) in kilotons and megatons of TNT equivalence.
(IN 5 26/30)