In British English, 'ill' is the opposite of 'well'. If you feel ill, it means that you aren't well. If you feel sick, it means that you are nauseous and might vomit. If you say 'I was ill this morning' it means that you didn't feel well; if you say 'I was sick this morning', it means that you vomited. In American English, the usage is different. American English speakers often use them interchangeably, or use 'sick' to refer to minor short-term illnesses (such as flu or an upset stomach) and 'ill' for longer-term and more serious conditions. I'm sure that an American English speaker can give you more details about this, though.

''Sick'' is the less formal of the two words. It usually describes short-term diseases or ailments, like the flu, and is commonly used to refer to a feeling of nausea. but ''ill '' is more formal and is used to describe long- and short-term diseases or ailments.

They mean the same thing. Ill tends to be used more in England, and sick is more commonly used here in the States.

I would say they have the same meaning? Maybe sickness is when you get a cold, a flu, want to vomit or small things like that and illness tends to be worst like a cancer, AIDS. But I'm not a native speaker so I'm not sure!