SICK and ILL
Both of these words are most commonly used as adjectives, as in:
I feel sick.
Ed had to leave school early because he was ill.
In American English, these two words have more or less the same meaning – they refer to a feeling of being unwell, whatever the problem might be. On the other hand, in British English, they have more distinct meanings. Sick most commonly refers to feeling as if you are going to vomit, as in:
I think I’m going to be sick.
…while ill refers to any other feeling of being unwell. However, there are some phrases which are commonly used where sick has a more general meaning. For example:
I’ve been off sick for ten days – meaning I haven’t been to work/school for ten days because I’ve been feeling unwell.
Sick can also be used as a noun to refer to vomit. For example:
If you have children, you can be sure that you will have to clear up some sick at least once during their childhood.