As the new coronavirus continues to spread around the world, the words epidemic and pandemic are showing up in news reports more often than they usually do. While the terms are closely related, they don’t refer to the same thing.

As the Association for Professionals in Infection Control and Epidemiology (APIC) explains on its website, “an epidemic occurs when an infectious disease spreads rapidly to many people.” Usually, what precedes an epidemic is an outbreak, or “a sudden rise in the number of cases of a disease.” An outbreak can affect a single community or several countries, but it’s on a much smaller scale than an epidemic.

If an epidemic can’t be contained and keeps expanding its reach, public health officials might start calling it a pandemic, which means it’s affected enough people in different areas of the world to be considered a global outbreak. In short, a pandemic is a worldwide epidemic. It infects more people, causes more deaths, and can also have widespread social and economic repercussions. The spread of the Spanish influenza from 1918 to 1919, which killed between 20 and 40 million people around the world, was a pandemic; more recently, the H1N1 influenza created a pandemic in 2009.

Here’s where it gets a little tricky: There’s no cut-and-dried classification system for outbreaks, epidemics, and pandemics. Based on the definitions above, it might seem like the current coronavirus disease, now called COVID-19, falls into the pandemic category already—according to a map from the World Health Organization (WHO), there are more than 80,000 confirmed cases in 34 countries, and nearly 2700 people have died from the disease. It’s also beginning to impact travel, stock markets, and the global economy as a whole. But WHO maintains that although the situation has the potential to become a pandemic, it’s still an epidemic for now.

“It really is borderline semantics, to be honest with you,” Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, told CNN earlier this month. “I think you could have people arguing each end of it. Pandemics mean different things to different people.”