The phrase comes from the government's terrorism bill. It was in use at the time of the 2005 Queen's speech, when the intention to pass a new bill was announced, but it was not until after the July 7 London bombings that it became an established part of the political lexicon.

Six days after the attacks, Tony Blair said the new anti-terror laws would aim to "pull up this evil ideology by its roots" by tackling incitement to terrorism. He announced a two-week consultation on a variety of measures, including how the law against those "glorifying" terrorism could be tightened.

What happened?

Glorification - a person who "glorifies, exalts or celebrates" a terrorist act - made it into the draft legislation published the following September.

The legislation threatened a five-year jail sentence for anyone glorifying a terrorist act committed over the last 20 years. The small print said the home secretary would draw up a list of historical terrorist acts that it would be a criminal offence to glorify.

The following month, the list was abandoned, the jail sentence reduced to 12 months and the definition of glorification tightened up so it was necessary to prove the intention was to incite further acts of terror.

Even in its revised form, the offence did not get through the Lords when the upper chamber revised the legislation. Today, the government is attempting to win a vote that would reverse that decision and restore it to the bill.

What are the arguments?

In the Lords debate, the Home Office minister, Baroness Scotland of Asthal, told peers the government did not believe it "acceptable that people should be allowed to make statements which glorify terrorism".

She said ministers believed such statements made it "more likely their audience will themselves commit acts of terrorism".

Those who oppose the proposed offence argue that existing laws are already used to prosecute individuals such as Abu Hamza, who would appear to be targets of the glorification charge.

What is the alternative to glorification?

In striking down glorification, the Lords proposed an offence of "describing terrorism in a way that would encourage people to emulate it".

The Tories accuse Downing Street of manufacturing a "bogus" spat over the issue in order to make the prime minister look tougher on terror than his opponents. Dominic Grieve, the shadow attorney general, said the alternative wording proposed by the Lords actually made the legislation tighter.

The home secretary, Charles Clarke, said the Lords amendment referred only to words which might lead a listener to emulate terrorism, meaning placards and images such as those carried on the anti-cartoon protests in London would not be covered. He said amendment would allow the glorification of terrorism "with impunity".

What about free speech?

When the bill was first proposed, there were fears the offence of glorification risked criminalising such figures as the Irish taoiseach, Bertie Ahern, when he commemorated the 1916 Easter Rising.

In the Lords debate, Baroness Scotland made it clear that, under the revised form of the bill then under consideration, prosecutions would proceed only when it could be shown a suspect clearly intended that terrorist acts should be emulated.

Mr Clarke received unexpected support in this area today when the organisation representing university vice-chancellors said they were satisfied ministers had met their concerns about the section on intent.

Universities and academic unions had feared a history lecturer who quoted statements by revolutionaries or a politics class shown an Osama bin Laden video could face jail for glorifying terrorism.

The most common challenge to glorification (not entirely sidestepped by the sections on intent) is where it would leave the supporters of a figure such as Nelson Mandela and the ANC's struggle against apartheid-era South Africa, which used violence.

Geoffrey Bindman, a leading human rights lawyer, told the BBC's Today programme that the law could become a "dangerous inroad on freedom of speech" if it was used against people who said it could be necessary to use violence against a repressive regime.

What are the existing laws?

The laws currently available to prosecutors come from a number of acts. Hamza's conviction included four charges brought under the Public Order Act 1986 of "using threatening, abusive or insulting words or behaviour with the intention of stirring up racial hatred".

The first conviction of a Muslim preacher - that of Abdullah el-Faisal in 2003 - was also first use of the 1861 Offences Against the Person Act for soliciting murder without a specific victim (he called for the killing of Jews, Hindus, Americans and non-believers).

Existing terror laws - those of 2000 and 2001, which the current bill replaces - do not deal with such soliciting or incitement. Hamza's sole charge under the Terrorism Act was for the possession of a document, the Encyclopaedia of the Afghani Jihad, containing information "of a kind likely to be useful to a person committing or preparing an act of terrorism".