A medical instrument which consists of a couple of earpieces, a bell and/or diaphragm and tubing connecting it all together. It works by transmitting sound and enables the doctor or nurse to hear sounds at the other end of the stethoscope.
Essentially, a stethoscope exists for the purpose of eliminating the need for a doctor to directly place his/her ear on the patient to hear sounds. It just would not be elegant for a doctor to listen several times on a patients by leaning against the patient.
Use of the stethoscope is necessary for the auscultation bit of the physical examination.