What does acute on chronic mean?

Acute on chronic means an exacerbation of a chronic, almost asymptomatic condition.

Sudden decline at a faster rate of a known condition expected to cause decline at a slower pace, typically described in renal or cardiopulmonary conditions for example.

Acute condition arises suddenly and lasts for a short period of time. The flu, asthma ,bronchitis, common cold, pneumonia and heart attack. A chronic disease lasts a long time such as cancer, arthritis, diabetes, heart, asthma, glaucoma multiple sclerosis and many more. Both are treatable but the chronic lasting for years.

A chronic disease implies one which has persisted for 3 months or longer and may worsen over time. Acute-on-chronic implies a reversible worsening of the condition of a person with a chronic disease; the transient nature of the acute worsening may be due to spontaneous factors or it may be due to active treatment of the acute insult.

"Acute" means occurring suddenly or quickly, like a heart attack, whereas "chronic" refers to diseases or conditions that develop gradually and persist for a long time, like atherosclerotic disease. Some conditions like diabetes are usually chronic, but may have acute episodes, such as diabetic coma when blood glucose levels quickly become extremely high.