When is an epidemic considered a pandemic, and what is the difference? What do health officials mean when they recommend “self-quarantining” or “social distancing”?

As the coronavirus spreads around the world, new terms are entering the lexicon — and we’re here to help. Here’s a guide to the words and phrases you need to know to keep informed of the latest developments.

Pandemic

Last week, the World Health Organization declared the coronavirus outbreak a pandemic after it spread across six continents and more than 100 countries. A pandemic is the worldwide spread of a new disease that affects large numbers of people. The W.H.O. had avoided using the word before Wednesday because it didn’t want to give the impression that the disease was unstoppable.

“Pandemic is not a word to use lightly or carelessly,” Dr. Tedros Adhanom Ghebreyesus, the director general of the W.H.O., said at a news conference.