An abbreviation in
hospitals for "DO NOT RESUSCITATE."
A patient can fill out this form, signing it as a legal document stating that if they go into cardiac arrest or respiratory arrest then they do not wish to be resuscitated.
This means that if the patient stops breathing or his heart fails, the staff will not perform CPR, defibrillate or hook up to a respirator or life-support. The reasons are pain, not prolonging a painful and artificial life, and some religious reasons.
Some family members may object to this, but it's all up to the patient, and if the patient wants a DNR they must be allowed one, regardless of what the children or spouse says. If the patient is a minor, then it's up to the parent or legal guardian. If the child doesn't want a DNR, but the parents do, then the laws in the US are sorta unclear, and YMMV.
This does not mean the doctors won't do anything, they will still give antbiotics, operate, and everything else they can to keep the patient comfortable and alive. EMTs can't give the person respirations or CPR, but they will do everything else such as give oxygen or treat other wounds.
Some people carry around a DNR outside, in public, so if they collapse, they don't want help. If you don't know that and give him/her CPR and they live, they probably couldn't sue you because of the Good Samaritan Law (in the US), unless the DNR is written on their chest blatantly.