Elsevier created a COVID-19 resource centre in January 2020, offering free information in English and Mandarin. The centre is hosted on Elsevier Connect, and Elsevier grants permission for free access to its research in PubMed Central and other repositories. A clinical case definition for post-COVID-19 condition was developed through a WHO-led Delphi process involving 265 participants, including patients, clinicians, researchers, and WHO staff. The definition includes 14 domains and 45 items, with a final consensus definition for adults: post-COVID-19 condition occurs in individuals with a history of probable or confirmed SARS-CoV-2 infection, usually 3 months from the onset, with symptoms lasting at least 2 months and not explained by an alternative diagnosis. Common symptoms include fatigue, shortness of breath, and cognitive dysfunction, and may be new or persist from the initial illness. Symptoms may fluctuate or relapse over time. A separate definition may apply to children. The study involved two rounds of Delphi exercises, with 265 participants in the first round and 195 in the second. The final definition was based on consensus, with 70% or more responses on a nine-point Likert scale. The definition is intended to standardize clinical case definitions for post-COVID-19 condition, facilitating research, policy, and clinical management. The definition is compatible with previous suggestions but may change as new evidence emerges. The study acknowledges limitations, including language bias and underrepresentation of older participants. The definition is not meant to replace other terms but to be used in a standardized way. The study highlights the need for ongoing research and the importance of considering diverse perspectives in defining post-COVID-19 condition.Elsevier created a COVID-19 resource centre in January 2020, offering free information in English and Mandarin. The centre is hosted on Elsevier Connect, and Elsevier grants permission for free access to its research in PubMed Central and other repositories. A clinical case definition for post-COVID-19 condition was developed through a WHO-led Delphi process involving 265 participants, including patients, clinicians, researchers, and WHO staff. The definition includes 14 domains and 45 items, with a final consensus definition for adults: post-COVID-19 condition occurs in individuals with a history of probable or confirmed SARS-CoV-2 infection, usually 3 months from the onset, with symptoms lasting at least 2 months and not explained by an alternative diagnosis. Common symptoms include fatigue, shortness of breath, and cognitive dysfunction, and may be new or persist from the initial illness. Symptoms may fluctuate or relapse over time. A separate definition may apply to children. The study involved two rounds of Delphi exercises, with 265 participants in the first round and 195 in the second. The final definition was based on consensus, with 70% or more responses on a nine-point Likert scale. The definition is intended to standardize clinical case definitions for post-COVID-19 condition, facilitating research, policy, and clinical management. The definition is compatible with previous suggestions but may change as new evidence emerges. The study acknowledges limitations, including language bias and underrepresentation of older participants. The definition is not meant to replace other terms but to be used in a standardized way. The study highlights the need for ongoing research and the importance of considering diverse perspectives in defining post-COVID-19 condition.