LONDON — The World Health Organization on Wednesday published a definition of long Covid for the first time, seeking to provide much-needed clarity on one of the most mysterious aspects of the coronavirus pandemic.

Here’s how the U.N. health agency has defined long Covid, referring to it as “post Covid-19 condition,” the name proposed by WHO’s International Classification of Diseases.

“Post COVID-19 condition occurs in individuals with a history of probable or confirmed SARS-CoV-2 infection, usually 3 months from the onset of COVID-19 with symptoms that last for at least 2 months and cannot be explained by an alternative diagnosis,” the WHO said.

“Common symptoms include fatigue, shortness of breath, cognitive dysfunction but also others … which generally have an impact on everyday functioning. Symptoms may be new onset, following initial recovery from an acute COVID-19 episode, or persist from the initial illness. Symptoms may also fluctuate or relapse over time.”

The WHO said a separate definition may be applicable for children.

The global health agency has previously said it has taken a long time to resolve a formal definition of long Covid because there are so many linked symptoms associated with the condition.

“Within the whole of the WHO, this has been a major issue for us,” Dr. Mike Ryan, executive director of the WHO’s Health Emergencies Program, said on Wednesday.

“We have to remain vigilant, this pandemic is not over and it continues to cause disease, continues to cause death, but it also continues to cause long-term consequences for people around the world,” Ryan said. “Again, within the falling numbers that we see, that masks tremendous problems emerging in countries.”

Ryan described the definition of long Covid as “a great step forward,” before adding the WHO’s understanding of the condition was evolving and therefore likely to change.