"At death's door" is an idiom used to describe a person who is very ill and close to dying. There are several synonyms for this phrase, including "critically ill," "terminally ill," "on the brink of death," "on death's doorstep," and "gravely ill." These phrases are often used to describe the severity of an illness or injury and the urgency of medical treatment. It is important to note that while these phrases may be used interchangeably, they each convey slightly different connotations and should be used appropriately in context. Regardless of the specific words used, it is always important to seek professional medical help if someone is experiencing health complications.