Oxford Dictionaries this week chose "post-truth" as its new word of the year. It could instead have opted for an old term that is very much in use in 2016: "fascism".

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Anti-immigration sentiment and rhetoric on both sides of the Atlantic have been labelled fascist, and critics of Donald Trump, some of them well within the political mainstream, have suggested that his victory in the US election is a sign that fascism is on the rise there.

Nigel Farage and Ukip are often called fascist. Even the pressure exerted on those in the public eye to wear a poppy commemorating Britain's war dead has been dubbed "poppy fascism".

Other commentators have resisted the use of the term, and suggested that it does not accurately describe today's politicians in the US and UK.

What does fascism mean?

The Oxford English Dictionary says fascism is "an authoritarian and nationalistic system of government and social organisation which emerged after the end of the First World War in 1918, and became a prominent force in European politics during the 1920s and 1930s, most notably in Italy and Germany".

The concept is a complex one and it can sometimes be easier to point to examples – such as the regimes of Benito Mussolini, Adolf Hitler or Francisco Franco – than offer an all-encompassing definition.

As secondary school history teachers have explained for decades, the word came into English from Latin, via Italian. The "fasces" were a bundle of sticks used to administer punishments, which became symbolic of government in ancient Rome. They were seized upon as an emblem by Mussolini as he tried to drag 1920s' Italy back to the perceived glory days of the Roman Empire.

So Mussolini created the term originally?

Yes - but by extension, says the OED, the word can now mean any "extreme right-wing political ideology based on the principles underlying [Mussolini's and Hitler's regimes]".

Mussolini summed up his politics in three clauses in a speech he made in 1925: "Tutto nello Stato, niente al di fuori dello Stato, nulla contro lo Stato." ("Everything in the state, nothing outside the state, nothing against the state.")

Are Trump and Farage really 'fascists'?

Clearly not. Neither has ever espoused fascist government. But the charge is levelled at them on the basis of the "thin end of the wedge" argument: they are both nationalists and have both focused on immigration. Nationalism is integral to fascism - and the scapegoating of immigrants is deeply associated with it.

For example, opponents claim there are echoes of the rise of Hitler's fascist regime in Trump's demonisation of Muslims. The Nazis chose to scapegoat Jews to distract the German populace from their real problems – and critics have argued that Trump has used Muslims and Mexicans for exactly that purpose on the campaign trail.

It would be harder to pin that charge on the Ukip interim leader, whose rhetoric has been more moderate. But some liberals point to his insistence that the presence of immigrants in the UK is a major cause of social problems and to his admiration for Trump.

Others argue that to use the label "fascist" to describe politicians such as Trump and Farage trivialises the crimes of real fascist regimes and those who suffered at the hands of their dictators.