The World Health Organization (WHO) declared the novel coronavirus (COVID-19) a global pandemic on Wednesday, but before WHO had been using the term "epidemic." What does this mean?

According to the WHO official website, there are six classification stages describing the evolution process of a novel virus, from phase 1 with "no reports of animal influenza causing human infections" to phase 6 of "a pandemic."

First, when a disease infects some people in a particular place in a short period, it's called an outbreak. As the outbreak spreads rapidly in various locations it becomes an "epidemic."

WHO defines an epidemic as "the occurrence in a community or region of cases of an illness ... clearly in excess of normal expectancy," which is a term that has been used frequently.

When an epidemic is widely spread in various areas across the world, crossing international boundaries, it becomes a global pandemic. Pandemic is described as "an outbreak of a new pathogen that spreads easily from person to person across the globe," according to the WHO definition. 

Pandemic refers to the spread of a disease and not its potency or deadliness, theoretically.

"This is the first pandemic caused by a coronavirus," WHO Director-General Tedros Adhanom Ghebreyesus said at a media briefing. "In the past two weeks, the number of cases of COVID-19 outside China has increased 13-fold, and the number of affected countries has tripled.”

The outbreak of Severe Acute Respiratory Syndrome (SARS), which is also a coronavirus, happened between 2002 and 2003 but was contained enough to avoid being classified as a pandemic.

The last time the WHO declared a pandemic was during the 2009 H1N1 swine flu outbreak, which is known as "endemic" worldwide today. 

Read more: Answer Bank: How can other countries learn from China's fight against COVID-19?