Sick is the less formal word, and it usually describes a more temporary health
problem – often nausea, although not always.
▶️ If you’re feeling sick, you might need to vomit.
▶️ If you get the flu, you might be sick for two weeks.
▶️ If a child is sick, he stays home from school that day.
▶️ Taking a day off from work for health reasons is called “taking a sick day.”
Ill is the more formal word, and it usually describes a more serious, more
permanent health problem, like cancer.
▶️ My uncle has been ill for the past five years.
▶️ She passed away (=died) after a long illness.
▶️ Someone with a mental health problem is mentally ill.
▶️ A disease that will definitely result in death is a terminal illness.