A geriatrician is a doctor whose work is the medical treatment and care of old people. The field is geriatrics; the geriatrician is the person. They do not look after only one body part. They look at how several illnesses, medicines, falling, memory, and daily life fit together in later life. It is a hospital and clinic word, quite formal. In ordinary talk someone might say “a doctor for older people.” This is not the same job as a gerontologist: that person studies ageing as science and may never give medicine.