Which term signifies a state of not feeling well?

The term "ill" specifically describes a state of not feeling well and encompasses a range of health issues, from mild discomfort to more serious medical conditions. It is often used in a medical context and directly connotes a negative state in terms of health.

While "sick" also conveys a similar meaning, it can sometimes refer to nausea or vomiting, which narrows its application compared to "ill." "Weak" and "unwell" can indicate a general state of reduced health or vitality but do not explicitly define the condition as being unwell in the same straightforward manner as "ill." Therefore, "ill" is the most precise term for signifying a broader state of not feeling well.