COVID-19, has officially been declared a "pandemic" by the World Health Organization (WHO), which has opened a debate on the meaning and usefulness of the word. 

"We are deeply concerned both by the alarming levels of spread and severity and by the alarming levels of inaction. We have therefore made the assessment that COVID-19 can be characterized as a pandemic," the WHO's director-general, Tedros Adhanom Ghebreyesus, told a news conference on Wednesday.

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While he acknowledged the distinction did not change either what WHO was doing or what countries needed to do, it sounded an alarm the organization had put off.

WHO officials signaled for weeks that they may use the word "pandemic" as an descriptive term, while repeatedly stressing that it does not carry legal or economic significance. The WHO no longer has a category for declaring a pandemic, except for influenza.

In practice, the word has been used to describe significant geographical spread of a disease or virus over a short period of time. Usually, it is applied when a disease spreads across a wide region, perhaps several continents. A disease that quickly infected every single person in Denmark or Denver would be awful, but it wouldn't be a pandemic: it would, however, be an epidemic.

Again, like pandemic, it is a term without a clear definition. The Ebola crisis of 2013-16, which primarily affected the neighboring West African countries of Liberia, Sierra Leone and Guinea, was widely described as an epidemic; however, a 1,750-word statement from the WHO on 7 August 2014, describing it as an "extraordinary event" and a capitalized "Public Health Emergency of International Concern," did not once mention the word "epidemic." Only in September, as the case toll climbed, did the WHO begin to use that word. The WHO later faced criticism for the speed of its reaction to the crisis.

An epidemic grows from an outbreak, which can be described as a sudden spike of cases in a localized region – above what may normally be expected (for instance through a typical seasonal variation, as with malaria). With an unknown disease, as few as four cases may be regarded as an outbreak.

A cluster is something of a preliminary term: an accumulation of cases in close proximity and time, which experts suspect is greater than the expected number. Such an occurrence would probably remain a cluster while tests are carried out – and time passes – before possibly escalating to an outbreak, then an epidemic, then a pandemic.

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However, the distinctions between these words are at best loosely defined. U.S. National Institute of Allergy and Infectious Diseases chief Anthony Fauci declared earlier this month: "Pandemics mean different things to different people …  It really is borderline semantics, to be honest with you."

Wary of inflaming worldwide worry, authorities are being very careful with language. They did not rush to name the new coronavirus and were careful to do so without linking it to a place or animal, which can increase fear – such as happened with MERS (Middle East Respiratory Syndrome, a name that inspired mistrust of people from that region) or swine flu (during which outbreak, Egypt slaughtered all its pigs).

As Ghebreyesus insisted previously: "Using the word 'pandemic' carelessly has no tangible benefit, but it does have significant risk in terms of amplifying unnecessary and unjustified fear and stigma, and paralyzing systems. It may also signal that we can no longer contain the virus, which is not true."

That his position has now changed, only highlights the seriousness of COVID-19.