The outbreak of the novel coronavirus that’s now sickened more 100,000 people across the world has officially been declared a pandemic by the World Health Organization (WHO) — weeks after WHO chief Tedros Adhanom Ghebreyesus told reporters in February that the outbreak of the virus, and the disease it causes --COVID-19-- “absolutely” had the potential to become a pandemic.

"WHO has been assessing this outbreak around the clock and we are deeply concerned both by the alarming levels of spread and severity and by the alarming levels of inaction," Tedros said during a Wednesday press conference. "We have therefore made the assessment that COVID-19 can be characterized as a pandemic."

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But what is a pandemic?

Epidemiologists — simply put, those who study disease outbreaks in human populations — typically look at disease prevalence, incidence, and either known or unknown disease pathways, among other factors, when describing a disease event, according to Verywell Health. The terms “sporadic,” “cluster,” “endemic,” “hyperendemic,” “epidemic,” “outbreak,” and “pandemic,” are often those used to describe different disease events.

To understand a pandemic, however, it’s important to first know the term epidemic — or what one report defines as an “outbreak that spreads over a large geographical area.” The Zika virus that began in Brazil in 2014, the Ebola outbreak in West Africa that began the same year, as well as the ongoing opioid crisis in the U.S., are all considered to be examples of an epidemic, which is also described as a problem “that has grown out of control,” according to Verywell Health.

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A pandemic, on the other hand, “refers to an epidemic that has spread over several countries or continents, usually affecting a large number of people,” according to the Centers for Disease Control and Prevention (CDC). The 1918 Spanish influenza is perhaps the most infamous example of a pandemic. The 2003 outbreak of Severe Acute Respiratory Syndrome (SARS) — which also began in China — is also considered to be a pandemic.

"Scientists use the term 'pandemic' to describe a new virus that emerges and spreads to multiple countries throughout the world. It means that the new virus is widespread and is spreading efficiently in those countries, but it does not tell us how severe the virus may be," Dr. William Schaffner, the medical director of the National Foundation for Infectious Diseases (NFID), previously told Fox News in an email, noting WHO is ultimately responsible for designating a pandemic, as it did on Wednesday.

Some of the criteria for a pandemic:

Describes the worldwide spread of a new disease

Involves a new virus against which most people don’t have immunity

Virus can infect people easily and spread from person to person

Implies efforts to contain an outbreak in a region our country have failed

A pandemic describes how widespread an illness is, not how lethal it is

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Additionally, there are different phases of pandemics. For instance, the WHO has a pandemic alert system for the flu, ranging from Phase 1 to Phase 6. Phase 1 means there is a low risk of a flu pandemic, while Phase 6 which refers to a “full-blown pandemic.”

"The advice is still the same: Wash your hands thoroughly, avoid sick people, and stay home if you are ill," added Schaffner.

Fox News' Brainroom contributed to this report.