Definition of ill adjective from the Oxford Advanced Learner's Dictionary

Oxford3000

ill

adjective
ɪl
 
; ɪl
 
 
1 (especially British English) (North American English usually sick) [not usually before noun] suffering from an illness or disease; not feeling wellHer father is seriously ill in St Luke's hospital.She was taken ill suddenly.We both started to feel ill shortly after the meal.Uncle Harry is terminally ill with cancer (= he will die from his illness).the mentally ill(= people with a mental illness) (formal) He fell ill and died soon after. see also illness2 [usually before noun] bad or harmfulHe resigned because of ill health(= he was often ill).She suffered no ill effects from the experience.a woman of ill repute(= considered to be immoral)3 (formal) that brings, or is thought to bring, bad lucka bird of ill omen
Idioms

ill at ease

feeling uncomfortable and embarrassed
I felt ill at ease in such formal clothes.

it's an ill wind (that blows nobody any good)

(saying) no problem is so bad that it does not bring some advantage to somebody
more at bad/ill feeling at feelingUsage noteUsage note: IllnessesBecoming illcatch a cold/an infectious disease/the flu/(British English) flu/pneumonia/a virus/(informal) a bugget (British English) ill/(North American English) sick/a disease/AIDS/breast cancer/a cold/the flu/(British English) flu/a migrainecome down with a cold/the flu/(British English) flucontract a deadly disease/a serious illness/HIV/AIDSbe infected with a virus/a parasite/HIVdevelop cancer/diabetes/a rash/an ulcer/symptoms of hepatitishave a heart attack/a strokeprovoke/trigger/produce an allergic reactionblock/burst/rupture a blood vesseldamage/sever a nerve/an artery/a tendonBeing illfeel (British English) ill/sick/nauseous/queasybe running (British English) a temperature/(North American English) a feverhave a head cold/diabetes/heart disease/lung cancer/a headache/(British English) a high temperature/(North American English) a feversuffer from asthma/malnutrition/frequent headaches/bouts of depression/a mental disorderbe laid up with/(British English) be in bed with a cold/the flu/(British English) flu/a migrainenurse a cold/a headache/a hangoverbattle/fight cancer/depression/addiction/alcoholismTreatmentsexamine a patientdiagnose a condition/disease/disorderbe diagnosed with cancer/diabetes/schizophreniaprescribe/be given/be on/take drugs/medicine/medication/pills/painkillers/antibioticstreat somebody for cancer/depression/shockhave/undergo an examination/an operation/surgery/a kidney transplant/therapy/chemotherapy/treatment for cancerhave/be given an injection/(British English) a flu jab/(North American English) a flu shot/a blood transfusion/a scan/an X-raycure a disease/an ailment/cancer/a headache/a patientprevent the spread of disease/further outbreaks/damage to the lungsbe vaccinated against the flu/(British English) flu/the measles/(British English) measles/polio/smallpoxenhance/boost/confer/build immunity to a disease